Missile fire v. battlements

[haakon1]

I'm coming at this from a 3.5e perspective, but thoughts from any edition/Pathfinder, or just general medieval military knowledge, would help.

The situation in my game is a battle for a castle. There's a ballista on top of the donjon (50 ft. tall battlemented tower) and the PC's want the troops they are leading to fire at the ballista crew, but from behind a building that offers cover relative to the ballista.

Here's my thoughts:

-- Ballista is essentially a Direct Fire Weapon, analogous to an anti-tank gun or a machinegun. It fires straight on, not with a ballistic trajectory, so it can't hit archers hiding behind a building. And from the roof, it would have limited ability to shoot DOWN anywhere near the tower, because it would need to be jacked up to point the "muzzle" down, and there's a limit to how much that can be done.

-- Longbows are combination Direct Fire/Indirect Fire (ballistic trajectory) weapons. That is, you can fire up to get a plunging fire which is inaccurate, but can easily go over walls (like mortars and howitzers in modern warfare, or catapults and trebuchets in medieval warfare).

-- Crossbows are Direct Fire Weapons.

http://en.wikipedia.org/wiki/Direct_fire
http://en.wikipedia.org/wiki/Indirect_fire

Looking at combat modifiers (3.5 PHB, p. 151-2), I'm thinking:

-- Ballista fire at archers/crossbowmen behind a building: Impossible, as there's no line of effect and it's a direct fire weapon

-- Ballista fire at archers/crossbowmen in the open: No penalty or bonus.

-- Crossbow or bow fire from the open ground at ballista spotter on the edge of battlements: AC+8 (double cover, like an arrow slit)

-- Crossbow fire at ballista loaders (back from the battlements), either from open ground or behind a building: Impossible, as there's no line of effect and it's a direct fire weapon.

-- Longbow fire at ballista loaders (back from battlements), either from open ground or behind a building: Total Concealment, so pick a square and if it's inhabited, 50% miss chance. Also, AC is +8 for excellent cover/penalty for indirect fire.

Sound about right, or do you have other ideas?

 

[MarkB]

There's nothing preventing a crossbow being pointed skywards and used for indirect fire. That's potentially even true of the ballista, if its mounting allows it.

Also, how sturdy is the building the archers are using as cover? As a siege weapon, the ballista may be able to damage it directly.

Shooting down may not be necessary for the defenders - anyone closing to below the ballista's line of fire is likely close enough to be attacked with thrown or dropped weapons.

Finally, are the ballista crew the only ranged combatants in the castle? It seems likely that the defenders would respond by assigning them some archers for defence in this situation.

 

[delericho]

My recommendation would be to just use the cover rules. Although the characters may be "behind" the building, they'll generally have to step out at some point to either take their shot or to spot their targets, and the ballista may well target them in that point.

It will look a bit odd on the battlemat, but it's pretty much an artefact of the round-based structure in D&D and its strict, discrete turns.

 

[haakon1]

There's nothing preventing a crossbow being pointed skywards and used for indirect fire. That's potentially even true of the ballista, if its mounting allows it.

Nod, I guess that makes sense. A ballista being capable of firing on a ballistic trajectory makes sense. But I don't think it could easily do full plunging fire like a mortar, so if they are close enough to the building, they are safely out of the arc of possible fire.

Also, how sturdy is the building the archers are using as cover? As a siege weapon, the ballista may be able to damage it directly.

Two story stone stable with a tile roof. The battle won't last enough for the ballista to do more than set the roof on fire, because the PC's are going to storm the door of the donjon. So it will be only a few rounds before it's hand-to-hand in the tower. The PC's victory is already a known conclusion from where I sit, but the PC's don't realize the enemy leaders have already escaped after doing what they came to do, and the guys left behind are just a covering force.

Shooting down may not be necessary for the defenders - anyone closing to below the ballista's line of fire is likely close enough to be attacked with thrown or dropped weapons.

Finally, are the ballista crew the only ranged combatants in the castle? It seems likely that the defenders would respond by assigning them some archers for defence in this situation.

It's late in the battle. The forces left for a last stand/to look to the PC's like they won are:

Dungeon: 2 Owlbears (CR4) running loose, to slow down the recovery of POWs in the cells. The enemy left the POW's alive only because one is a mole who will provide false information if the PC's believe them, but the enemy doesn't know the PC's already suspect him.

Donjon roof:
-- Warrior 3 Lieutenant, 5x Warrior 2 Ballista crew. All veteran fanatics for the cause (The Horned Society).
-- 2 Ogre Mages (CR8) currently invisible. Mercenaries, will kill for fun and flee if threatened, possibly "choppering out" remaining a Warrior or 2 each.
-- 2 Erinyes (CR8) acting as snipers. Fanatical devils, will flee once the battle is lost, carrying the most useful Warriors left alive.

First floor, behind a barricaded door and additional overturned trestle tables for cover:
-- 8x Warrior 2 with Alchemist's Fire, Light Crossbows, and Halberds, dressed as the castle guards though they know the jig is up on that ruse. Fanatics, will likely fight to the last man to make the PC's pay. These folks are refugees who have been running away for 1000 miles from Iuz's hordes, and will no longer run away from death. Think of Soviet Red Guards whose villages had been destroyed by the Nazis . . . no surrender, no retreat, no quarter. The fact that PC's also don't like Iuz matters nothing to them -- they or their appointed enemy must die, there is no other way for them.

The PC's are a group of 8, mostly 5th-7th level. The survivors of their force are about 100 men, mostly militia light crossbowmen and some mercenary longbowmen (all Warrior 1), with the rest spearmen (Warrior 1) or sappers (Expert 1).

Later today, the PC's and their forces will face the ACTUAL attack on the city that taking its small and not terribly well-designed castle was just a diversion for . . . well, let's just say I read this and modified: http://paizo.com/products/btpy80hm?...elords-Chapter-4-Fortress-of-the-Stone-Giants

Ah, and since I'm doing this slowly over email in a very long-running campaign, I actually do roll every single shot of a crossbow, using real normal 3.5e rules. Lucky matters more than you'd think. The opening volley of 80 crossbowmen included 13 19's or 20's rolled, so they did well against the enemy who were standing up from the battlements to pour fire on the battering ram party.

 

[haakon1]

My recommendation would be to just use the cover rules. Although the characters may be "behind" the building, they'll generally have to step out at some point to either take their shot or to spot their targets, and the ballista may well target them in that point.

It will look a bit odd on the battlemat, but it's pretty much an artefact of the round-based structure in D&D and its strict, discrete turns.

They are going for indirect fire -- hoping sheer volume of essentially blind firing at a known, fixed location target (roof top 50 ft. up and over the building, ~80 ft. thataway) will be effective enough to keep the enemy busy, and maybe get some lucky hits.

It's a valid tactic ('cause I care about real life more than rules), but I'm trying to figure out the rules. Cover should be very, very good, and chances to hit pretty low.

 

[MarkB]

They are going for indirect fire -- hoping sheer volume of essentially blind firing at a known, fixed location target (roof top 50 ft. up and over the building, ~80 ft. thataway) will be effective enough to keep the enemy busy, and maybe get some lucky hits.

It's a valid tactic ('cause I care about real life more than rules), but I'm trying to figure out the rules. Cover should be very, very good, and chances to hit pretty low.

I know there are rules for massed volley fire somewhere, but offhand I'd suggest modelling an indirect-fire volley at a known location as an area attack.

Maybe nominate a 'lead' archer whose attack bonus will set the DC of defenders' reflex saves, or else use the average of the attackers' rolls, and have the attack deal damage as per a single bow, with an extra die of damage added for every five attackers.

That's just off the top of my head - it will need some playing around with to see what's balanced.

 

[Grogg of the North]

The forces left for a last stand/to look to the PC's like they won are:

Donjon roof:
-- Warrior 3 Lieutenant, 5x Warrior 2 Ballista crew. All veteran fanatics for the cause (The Horned Society).
-- 2 Ogre Mages (CR8) currently invisible. Mercenaries, will kill for fun and flee if threatened, possibly "choppering out" remaining a Warrior or 2 each.
-- 2 Erinyes (CR8) acting as snipers. Fanatical devils, will flee once the battle is lost, carrying the most useful Warriors left alive.

Silly question. Why don't the Ogre Mages drop their cone of cold on the archers? The two of them could wipe out most of the foot soldiers and the Erinyes can easily pick off the stragglers. It seems to me that the crew on the roof can keep on doing what they're doing and let their flying friends deal with the ground troops.

 

[Umbran]

-- Ballista is essentially a Direct Fire Weapon, analogous to an anti-tank gun or a machinegun. It fires straight on, not with a ballistic trajectory, so it can't hit archers hiding behind a building. And from the roof, it would have limited ability to shoot DOWN anywhere near the tower, because it would need to be jacked up to point the "muzzle" down, and there's a limit to how much that can be done.

If the thing doesn't really work from on top of the tower, why is it there? Why don't they put a weapon that does the job on top of the tower? And what numbskull allowed a building that provides cover to be built within crossbow range of the tower? The whole point of a tower it to command territory around it, which it cannot do if the field of fire is cluttered with stuff.

Or, why don't they put the ballista on a universal joint (as was done for ballistae in the real world, once the universal joint was invented), so they can aim it where they want? In the d20srd, the thing already has a -4 modifier for a medium sized creature to aim it. I don't see a need to further hamper the thing.

The ballista is described as a big honkin' crossbow, with even the same range increment as the heavy crossbow. Whatever a crossbow can do, a ballista does, with a larger projectile. I'd say that if the crossbowmen can hit them, the ballista can hit the crossbowmen. And it only requires a crew of 1 to operate it - you might want to improve the aim or rate of fire if you're having a crew of several.

 

[haakon1]

Silly question. Why don't the Ogre Mages drop their cone of cold on the archers? The two of them could wipe out most of the foot soldiers and the Erinyes can easily pick off the stragglers. It seems to me that the crew on the roof can keep on doing what they're doing and let their flying friends deal with the ground troops.

Because they are still staying invisible for now. The PC's don't know what they are up against, and that's how the enemy likes it. Always leave your enemy guessing as to your capabilities. After all, the enemy is a secret society that supposedly were destroyed.

Once the PC's are in the donjon, the ogre mages will "decloak" and destroy the militia and town guards at will, or so they think -- the PC's actually gave some magic crossbow bolts and arrows to some of the militia, in case of such an eventually. The PC's, with authority from the local Bishop of Pelor and the lower ranking Town Guard and Militia leaders who were the only leading town folks not "purged" before the PC's figured out what was going on, gathered all the magic items in the city, to spread about the remaining loyalist forces.

The PC's were worried about gargoyles, which they knew the enemy had, and the enemy tried to use them early on (along with summoned monsters). But the PC's made quick work of them, so the bad guys learned the lesson to wait for the PC's to separate from the troops, if possible, before dropping the big scary monsters.

 

[haakon1]

Umbran, the reason I started the conversation was to get advice on how this should work. If I had it all figured out beforehand, I wouldn't have asked.

I've done a fair amount of reading on this topic in the last few days, and this passage, from an Osprey book about the Battle of Crecy, seems to cover the topic well. This military historian, at least, agrees with my gut notion that a longbow is good at indirect fire, while a crossbow is not. At the Battle of Crecy, English longbowmen defeated elite Genoese heavy crossbowmen (and French knights). Traditionally, a lot of the story has focused on the rainstorm just before the battle. Supposedly, the longbowmen unstrung their bows to keep the strings dry, whereas with a crossbow, special gear is needed for that, so the Genoese crossbow strings got soaked and were weakened. But the Osprey historian thinks that's not the whole story:

In ordinary circumstances the only advantages that a longbow had over its more sophisticated opponent was the rapidity with which it could be shot, and its ability to rain heavy arrows from high trajectory. In terms of accuracy, range and penetrating power, the advantage should have lain with the Genoese crossbows. When the Genoese did shoot they had to do so uphill with a low sun either in their eyes or slightly to their left. This was a particular disadvantage for men who aimed directly at their targets rather than dropping arrows on them in showers.

So yes, crossbows are primarily direct fire weapons, like rifles. Whereas longbows are often used for indirect arcing trajectories, like mortars.

Reading up on Roman ballistae, I discovered they were typically used for direct fire -- shorter range but highly accurate for sniping -- but could also be used for indirect arcing fire -- to hit at longer distances.

Umbran is correct about the use of universal joints in ballista, which would allow the switch from direct fire to indirect to be quick and easy.

For rules, I’ll went with this:

Longbows -- firing from behind a two-story building, at the top of a 50 ft. tower behind it:
-- Must guess target square correctly. If guessed correctly, 50% miss chance (Total Concealment)
-- Enemy AC+4 for not being able to see the flight/landing area of their shots.
-- Enemy AC+2 for range (120 ft. laterally plus 50 ft. vertically, on a 110 ft. range increment
-- Since the enemy are AC14, this works out to AC20 with a 50% miss chance. With the Warrior 1 Longbowmen having +2 TH, this means they need to roll a natural 18 or higher, plus a 50% miss chance, if they pick the correct square to target.

Light Crossbows-- firing from behind a two-story building, at the top of a 50 ft. tower behind it:
-- Must guess target square correctly. If guessed correctly, 50% miss chance (Total Concealment)
-- Enemy AC+4 for not being able to see the flight/landing area of their shots.
-- Enemy AC+8 for range (120 ft. laterally plus 50 ft. vertically, on a 80 ft. range increment, but I'm halving the range increment to 40 ft. (and rounding down) because the crossbow is being used for indirect fire, which it's not designed for)
-- Since the enemy are AC14, this works out to AC26 with a 50% miss chance. With the Warrior 1 Crossbowmen having +2 TH, the shot is not possible for them.

Ballista-- firing from the top of a 50 ft. tower, and people clinging to the backside of a two story building
-- Same modifiers as for longbow, except for range increment penalty. So 50% miss chance, and enemy is AC+4 for not being able to see the flight of/landing area of the shot.

One thing I'm not sure on, is whether people at the very base of the two-story building should be immune from ballista fire and unable to hit the tower with arrows. That makes sense to me, because there has to be a limit to how vertical a shot can climb the first/last part and still get in the right trajectory to get across the gap.