Exit the Keep and save your game when you're outside. Now quit the game. Open Windows Explorer, find the directory that has the game in it, and in there, look for the "minisaves" directory. Open the minisaves directory, and delete every file in there that begins with "Anskramkeep". The close Explorer, reload the game, reload your savegame, and enter the building from the back as Christy advised. You should now be able to complete the Keep without problems - apart from those caused by the monsters and traps inside, of course!