Laterali: You either find the drunk that those 3 guys are yelling for in henselt's camp or you help the veteran's son defeat the guy in the tournament. If you ask the right questions they give you a coin and instructions on getting into the lower level of the tower., where the plotters are.
vindik8or: Uhh, no duh. I'm talking about the door that says 'key required'
My assumption is that the game, like virtually any rpg, has content that simply never made it to the final version. Hence a few locked doors that don't ever seem to unlock throughout the game. Since there are also doors that you simply can't even interact with, one assumes that any locked door was originally placed for a purpose.
I certainly could have missed something, and the sounds down there sure seem to indicate there is more going on, but I never found a way to open that particular door either.