Last week I played a very interesting session. We were in a small town that was being overrun by a large army of orcs commanded by their drow masters. Some of it worked and some of it didn't, so I'll give a bit of an action report:
The DM had drawn up a small scale map of the village. Initially, we just noted what house we were staying in when the assault began, but very quickly, the party split up. Our archer was hit by a drow arrow as soon as he stepped out the door and fell to the sleep poison. (If only he'd taken fighter levels instead of Order of the Bow Initiate, he would have passed--though he'd probably have passed with a good cloak of resistance too

. The rogue and bard stepped out and were immediately hit by a glitterdust. My character dragged the archer back into the building and hid in the shadows as he went out. (You see, if you use the hide skill, they have to spot you and with the distance modifiers for 200 feet of distance, that's pretty difficult even if you have no hide skill at all (and my character is pretty good at it)).
This is one of the things that didn't work. Placing drow archers in the tops of trees surrounding the village put more than a slight strain on believability. But once everyone said "I hide, taking the penalty for moving full speed as I duck between the shadows" they pretty much went away.
So, my character made it to where he could see the front gate and the militia massing (not that there was really a mass), but there was no way to see what was going on outside--no arrow-slits or anything. (So, make sure the fortifications are at least marginally competent--you don't need firing ports everywhere, but at least have a block-house and a few observation points and make sure the block house has line of sight to the gate. Even villagers in a dangerous D&D world should have access to someone with enough experience to know some of the basic requirements of fortification--someone probably served in an army or a mercenary company or something). Meanwhile, the other characters made it to the south end of the palisade where they could see large groups of orcs advancing with tower shields. They prepared to hold the inevitable breach as well as they could.
Then the orcs broke through. The DM had this take one round as they stowed their tower shields and pulled out greataxes on the way up and then power attacked through the wall. Really, it probably should take longer than that. (It would probably be good for building up tension as well). The druid summoned a lion that pounced (well actually, just attacked, grappled and raked) the first orc, killing him on a final critical. At this point we realize that, if it takes 55 or so points of damage to put down one orc and we can see over sixty, there's no hope of victory. Especially when three other orcs kill the summoned lion in one round. (That leaves four orcs in the initial group of eight who still were going to get to act if anyone is counting). The druid cast fog cloud over the area and retreated. The bard and the glitterdusted rogue found a house to hide in (hoping that the spell would wear off before orcs kicked in the door.
What worked at this point: The feeling of inevitability. We were supposed to lose--it was just a question of how many of us would escape and if anyone would try to make a valiant last stand. Once the first orcs made it through the palisade, it was pretty obvious we weren't going to win. However, this created some believability problems as well. The militia at the gate were being pushed back but the orcs didn't cleave through them like they cleaved through the lion. Now, the militia were obviously far inferior to us in skill, but they were holding off groups of orcs that would kick our party's collective tail. It made escape possible, but like I said, there were some believability problems. It might have been better to simply double the number of orcs and make them lower level. So the lion kills one orc and grapples another then all of the others combine to bring it down--or maybe the druid, dire wolf companion, and lion are able to hold out for two rounds--either way, we'll have to abandon ship once the druid runs out of lions or the militia is beaten and twice as many orcs start coming towards the rear. Anyway, if the DM had used more orcs but weaker ones, it would have been more believable that the militia could hold them back for a little while at the gates--there's only room for so many orcs after all.
Then, our characters got down to the nitty gritty of trying to escape. Some orcs had broken through the northern palisade and moved to the temple of Lathander where one of the priestesses and some of the townsfolk had holed up. My character made his way through the buildings, hiding, with the vague intent of using his once per day cold substituted lightning bolt from his sword (more or less what it is) on them and hoping they weren't as tough as the orcs at the south wall and that he'd be able to take a few people with him on his escape--either that or include them in the cold bolt and at least give them a less painful death than the orcs would. At this point, it became necessary to draw the section out in 5' scale to tell how many orcs he could line up (the DM tells me "you can get three orcs"; I reply "put them on the map--I'm pretty sure I'll be able to manage twice that many) and also to see how close I could sneak (and what route I would have to take) without any of them having unobstructed line of sight. So we did that. Meanwhile another PC whose player had been gone for a couple months showed up with a band of rangers and silenced the drow archers and made his way to the south wall. My PC saw one orc, took a chance on sneak attacking him to try to get closer and discovered that he was just as tough as the orcs at the south wall (so the cold bolt wouldn't have done any good anyway). So, I make my way back around the buildings trying to keep ahead of an ever-growing mob of orcs (and to make the one's following me cautious--fortunately, my PC has elusive target and spring attack and he can do that kind of thing--especially when he's being chased by orcs in banded mail). As I move, we discover that we need to basically draw the whole town in. Eventually, my PC makes it into the druid's fog cloud and out into the forest where he can hide pretty effectively. About half-way through the chase, the PC slept by the drow arrow is coup de graced by an orc who kicks in the door and finds him there, and the other PCs come out of the building (the glitterdust wore off) and make their way to the southern wall.
This last bit worked pretty well (and probably would have worked even better with somewhat weaker orcs--my PC could have left a few bodies behind him rather than just some injured orcs and might have been tempted to make a last stand; that kind of thing is a whole lot less tempting when it's clear you'll be lucky to take one with you). It did show, however, that we should have drawn the entire town out in 5x5 scale at the start. (For a larger siege, this isn't practical, but you should certainly keep as much of it to scale as possible--if PCs split up or decide on different routes, it will be important exactly where they are and what challenges they will have to face to either get away or get to where they are needed).
So those are some lessons on what worked and what didn't from my last session.