You are attacking a rotted, ruined tower currently held by goblins.
When you enter, there's a standard "entry way, fight all the minions and stop them from sounding the alarm" type encounter. The only thing that's unusual is the fallen stone in the center of the room that some goblins use for cover.
The next fight is up one floor. Its terrain is the same size and shape as the floor below, because its just another floor of the tower. Except that stepping in the middle of the room causes it to collapse, dropping you back down to the first floor. That's right, don't put away the map for floor 1. You'll need it again. The fall is 20 feet, so that's 2d10 damage waiting for you at the bottom. Fortunately, you can throw the goblins down the hole once you've accidentally walked into it and opened it up.
The third floor is slightly different- it would be the same size and shape as the first and second, except half the wall is rotted away, along with parts of the floor. Falling on this floor will deposit you on one of the lower floors again, depending where you fell. That means somewhere between 2d10 and 4d10 damage. So keep those maps out! You'll need them again! Our party got to the top floor, collapsed the floor unknowingly by summoning a spirit companion bear right in the middle of the weak spot (we figured the bear has mass, it can bite people and it can't fly), and then started tossing goblins off the tower left and right. It was great.
Until my character died. But that was my/our fault. At current count, I can think of five different things I or the party should have done differently that would have still let me do the badass things I did without getting my head crushed by a bugbear's mace. I'm playtesting an Avenger, and when my Oath of Emnity target (the boss of the fight) was thrown off the tower and fell all the way to the ground floor, I followed him down by jumping cat-like from floor to floor in a single move action. I rocked it, landing on my feet and dealing him a serious blow to add to the 4d10 he had just taken. I was pretty sure I could take him, but unfortunately the party dropped a bugbear on my head. That hurt, and when it climbed to his feet and started flanking, things went from bad to worse very, very fast.