Thanks everyone for the advice!
The one encounter I haven't used from the adventure is the optional hobgoblins and wolves after Cragmaw Castle. I'll have Venomfang run into them while looking for allies. I'll upgrade one of them into a hobgoblin captain. With the intelligence of the dragon and the tactical skills of the hobgoblins, I can justify making some excellent fortifications.
I'm thinking of recruiting a band of orcs (with an eye of Gruumsh). I might make one of the hobgoblins into a shaman for a second low-level caster, since Venomfang herself already has her spells accounted for (
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suggestion. I'm strongly considering adding some combination of a couple of ettercaps, an ettin, or a troll. Since the party will likely have the ability to cast 4 fireballs, I'm probably not going to bother with kobolds and goblins. (The shaman might be a goblin.
I'm mostly likely going to have to run the encounter at her lair, since it would be too difficult for her to maintain reconnaissance of the entire area.
I've considered having the hobgoblins build fortifications, using orc labor to get it done quicker. Since she knows they have aerial capabilities (it was just a single
potion of flying before, but she isn't taking any chances), she would likely try to put a quick (non-flammable) covering over her tower. Rubble from the town can be used to make difficult terrain if I can figure out where it might be effective. The hobgoblins and orcs can build spiked pit traps in the ground also, but again, effective placement is going to be the trick. The minions are going to be able to extract about 10 doses of some venom from giant spiders, and I expect that can be put to good use on hobgoblin arrows.
There is enough rubble and trees around to create simple stacked stone structures that could house archers. Venomfang straight up watched the characters throw
fire bolts through the window of the garrison barracks in Thundertree to kill the zombies, so she's going to have the minions include some sort of obstacles that can be moved to block arrow slits from the inside when not in use. The trick will be finding a non-flammable material to build the cover on the top of the tower to defend against aerial assaults.
The hilltop is surrounded by thickets and trees, so you can't see the base of the tower and the cottage unless you are on the hill south of town, or you actually climb up the hill (I'll say that it goes from heavy obscurement and total cover, to 3/4 cover and light obscurement as you move further through the thickets towards the tower).
I really like the idea of softening them up and then herding them into a contained area of silence.
The problem I'm having is thinking up a way to encourage them to charge the tower.
With longbows, 2 or 3 hobgoblins could shoot out of the arrow slits at the top of the tower and get something like 8 rounds of attacks on the characters as they approach town. They can get 1/2 cover behind scattered trees every other round or so, but they are mostly just sitting ducks until they are close enough to hide behind the buildings and trees--and at that point, on up until they are part of the way up the hill to the tower, all they can see is the top of the tower.
Here's what I could see the party doing in that situation. After getting hit a couple times, they'd probably spend an hour or two cutting down a tree and making a portable wall they could hold in front of themselves as they approach. Attacking with flaming arrows might discourage that tactic. But the transmuter could turn the wooden wall into something non-flammable, so that counters that. Now the party has gotten to the edge of town, they've taken the time to heal from any arrow wounds, and they have a shield to protect themselves from further attacks while they are maneuvering through town to the hill (when they'll have to leave it behind).
Now they'll move up the hill through the thickets. I can have a troll or ettercap hiding, but they might pick the wrong side, and invalidate that tactic.
Once they can see the tower, I anticipate ranged spells. They might look for an opportunity to lob a fireball through an arrow slit.
I either need to create a situation where as soon as they are far enough through the thicket to see the base of the tower (or better yet, even sooner) they feel a strong desire to be up there. I suppose I could have minions light the thicket on fire. Hmmm. Yeah, that's a good idea. She really likes the thicket around her tower, but it shouldn't take long (in "dragon years") for a new one to grow. I'll have to soak it in some kind of flammable substance so it will go up quicker when they light it. That will help the effectiveness of the pit traps also. PCs moving at a slow pace are going to beat the DC 10 check to spot the trap automatically. If they are in action mode they won't be moving slowly and therefore won't have a check, meaning automatically falling into pits if they approach from the directions that have them. The PCs will likely notice the flammable substance, however, and may be able to figure out its nature. But there isn't likely to be much they can do about it. Or at least that means I need to have some way to guarantee they aren't going to waste time trying to "deflammify" the thicket.
A threat from down in the ruined thorp is ideal for that, but I don't think she really is going to have time to manufacture one (the PCs cleaned the area out already), unless the minions have some way of creating one. I'll look at the spell lists. It will have to be so good that they really think they need to get up to the tower and stop the dragon immediately, or they will just deal with the threat so they don't have to act on her timetable.
Alternatively, I could create an external diversion, that looks like a thinly veiled attempt to keep them
away from the tower, until they are far enough up to start the fires. If it looks like Venomfang is doing her darndedst to keep them off the hill...well, that would help.
Once they get up the hill, I'm not sure what to do. Without extensive reconstruction (which the hobgoblins don't have time for, especially using shoddy orc labor), the only place the PCs could be trapped inside is the tower itself, entered via the cottage. So I need to get them in there. I'm not sure the hobgoblins would approve of blocking the back door so there is only one ground level way in or out, but if I put a rope ladder that can be lowered from the top that might make them go along with it. There will be a (large) hatch to the top of the defensive covering, and Venomfang will not hesitate to use it if she has need of escape. It will be her primary door in and out anyway. I've already determined that walking up the stairs is 180', while Venomfang can breathe on them from near the top. I still haven't figure out what material would be available to make this covering with that they couldn't just set on fire, since stone roofs aren't something you can just throw together like you can stack some stones to make a temporary wall.
So I'll probably have the cleric and shaman up near the top of the tower so they can cast spells down and then take total cover on the old stone half-roof with the battlements while they maintain concentration. I'm not sure where to put the ettin. Either I'll have to have him crawl into the tower and wait for her there, or I'll have to use him outside for a softening fight. In either case, I don't want the PCs just taking a rest outside the tower before they come in.
I suppose I can have most of the orcs hiding somewhere just outside of town with instructions to start launching blind ranged attacks up to rain down on the top of the hill shortly after they see the thicket on fire. Perhaps that's where the ettin comes in. Half of the orcs can run up the the main path with the ettin, cutting off their retreat and perhaps encouraging them to come inside the tower. I'd need to find a way to still make it look like Venomfang is trying to keep them
out of the tower rather than in, however.
Nah, that whole orc thing isn't going to work. I should probably just stick them inside some crude stone structures at the top of the hill and have them rush out and fight any characters who make it to the top of the hill. Perhaps I can trap the inside of the cottage--nothing says "Stay Out" like traps.
Okay, well that's what I'm thinking. Other thoughts?