Durlag’s Tower?

Ok- I’m using BGT, I have Ntotsc installed. My normal game plan is to clear out all of the tower and level one of the basement except the warders, then come back later in an unmodded game. This time, I went to UB, DT showed up on my map, but unreachable from every neighboring territory. Does anyone know the trick to get there? I’m in chapter 3.