Entering via the front door doesn't work. To get the answer Select the hidden text between the highlights.
Highlights on Go round the back, break the weak point in the wall, (kill any monsters that interfere), enter one of the corner towers and use the stairs to climb to the level at the top of the wall, and enter the fort by the back door you will find up there. Start from there and clear the fort.
You may need to reset the fort. To do that, save your position outside the fort, exit the game, and use Windows Explorer to delete all files in the minisaves game folder that start AnskramKeep. Then restart the game, load your savegame, and the fort should have respawned.
HTH! Highlights off