We're currently on a short lull in gaming in Felltower, thanks mainly to my schedule (unavailable for three straight weekends.)
But I think it might be interesting to list of the connected moving parts that are giving my players fits in Felltower. I'm using their terms of the levels, here, irrespective of how I actually label them.
There are basically only two ways into the dungeon, now. The main entrance, and the tower entrance.
The PCs abandoned and lost their ladder-turned-bridge a few sessions back. The tower entrance is heavily built, and if it's locked from within it harms those who try to open it. They haven't found a way to destroy the entrance's lock.
There were at least two other ways in within the castle. The PCs destroyed the well entrance, so the orcs plugged it up with refuse and wreckage. It's clearable with a lot of manual labor that no one is willing to do. They haven't located where under the ruins the other one is, although they found those stairs when they found the Choke brothers.
The castle itself used to be an easy walk-in, over ruined walls and an open gate. They destroyed the gate in the down position (!), and the orcs repaired the walls to a sufficient extent that the PCs need to climb.
Outside the castle is the bugbear entrance, but that was filled with earth last time they checked. Someone or something collapsed it.
The Orcs. They've been relatively quiet, but they are there and potentially can attack the party as they limp home or if they futz around too much on level 1.
More orcs here - the "orc hole" is on this level. Orcs don't seem to patrol the area by the door to the Giant Fantastic Staircase, but they are close by.
The draugr are off of this level.
Stirges still hassle the PCs periodically thanks to an infestation near the connection to level 1.
There is a way in from outside, through the dragon's lair. But the whole of level 3 accessible from there is a series of dead-ends, connecting back to a heavily orc-held area of level 2. The orcs have put up multiple blockages to make even getting to them involve heavy digging.
The norkers and gnolls and ogres were cleared out. But the gnome (?) is still on the loose. The PCs have a key they believe is to a pair of massive doors, but can't find the keyhole. In addition, those doors are very close to the biggest complicating factor - The Lord of Spite.
There is a black reaver down on that level, which the PCs managed to trigger. They're not sure if it's patrolling around or not. That complicates getting to a number of areas nearby.
There is a sinkhole to a "massive treasure" but it sits on top of a mobile floor. The sinkhole is near some areas with monsters, too, so it's not an easy exploration.
The hallway near Phase Snake Junction has some obsidian golems in it, and the PCs can't figure out a way to clear them on the cheap (aka, one at a time, by attacking first.)
ALL of this is subject to the Lord of Spite. Although a lot of discussion centers around him "sleeping" in his "apartment," he moves around a lot. He could intervene in any of these situations, not just the one near his "apartment complex."
The PCs can get to this level by swimming and breathing water. The level is probably lethal for most of them.
Those seem to be the open complications.