it's a bug indeed. This place still buggeв in the game, so I dunno how to help you. Try to enter via different door, try to teleport in Battle Tower and back. There must be a script cutscene right when you enter into the guildmaster tower.
Btw, have you tried to visit William in a first time, in Broken Valley? If so - perhaps you already has a key?
I've tried all of that.
I dont think he has visited william before, if he did that already, william had already been dead, as I dont think he can protect william once leaving before battle tower, since a horde of black ring will auto attack once trying to go down the stairs after the conversation with william.
His case really seems bugged already I guess.
I was in that "area" before battle tower, but I specifically remember not going inside the house (I saved before going inside, got owned, and didn't go inside again after loading).
I can't finish the game.
