Here is an updated version of the Known Entrances of Felltower post.
As of: 3/12/2017
Bugbear Tunnels - a narrow entrance requiring crawling, from the surface ruins on top of Felltower. Current status: Partly blocked then re-cleared, known to the orcs, believed guarded or trapped.
Castle - not an entrance, but surrounds the main entrance.
Current status: Accessible only by climbing, as the PCs broke the main wood-and-iron portcullis in the down and locked position.
Dragon Cave Mouth - a mouth-shaped cave on the west side of the mountain, leads to the cavern warren roughly 100' or so below the first level of tunnels. Originally guarded by a fierce dragon. Current Status: Unguarded.
Note: The surface castle is only accessible by climbing or otherwise bypassing the walls. The orcs rebuilt the outer walls, the PCs utterly disabled the front portcullises in the locked and down position.
Main Entrance - fortified main entrance under the castle (pictured above). Current status: Guarded by orcs via the "pillboxes." Pillboxes are heavily shielded against magic and direct attack, so the PCs haven't yet been able to fully disable them. Requires entrance through the surface castle.
Orc Entrance - unknown location, but it's known the orcs enter the tunnels under Felltower via a tunnel, possibly from a cave-riddled canyon to the north.
Orc Hole - technically an exit, maybe. A hole that the orcs seem to come out of. May or may not have a surface entrance. Current status: In heavy use by the orcs.
Underwater Entrance - an underground inlet to the Silver River believed to reach deep into Felltower. Current status: unknown.
Tower Entrance - there is a "hatchway" entrance under the collapsed ruined tower knocked down by the PCs. Current Status: Often locked from within. Requires entrance to the surface castle of Felltower. Recently re-opened by the PCs.
Well Entrance - a secret entrance at the bottom of a dry well. Current Status: Inside the surface castle of Felltower, completely blocked up (filled top to bottom with trash, dirt, feces, etc.)
Gates - there is probably a gate from the Lost City of D'Abo. There is a gate to that city, it's possible you can come back through that or another gate.
There are rumors of other ways in, too, but they may have been identified already:
- the well. (almost certainly ID'ed.)
- the second surface entrance in the castle. (Probably the tower "hatch" or the bugbear tunnels.)
- possible entrances from other dungeons or caves far, far from Felltower. (In other words, find another dungeon, clear the sucker, and then march underground a long way, not even remotely identified.)
It's interesting that as a result of play, the PCs discovered a lot of entrances, but ended up getting them sealed off by the orcs (and others, possibly.) They have not been able to open up any of the previously open entrances that were sealed. Most of them involve being inside the gates in any case. There is still no convenient and safe entrance to Felltower.