Nope, I don't think so. I played 3 characters thru the original BG without ToSC and I can tell you there's no Durlag's Tower. Unless you're getting confused with the Firewine Bridge or Ulcaster's School.
I found out all about how the world map gets modified when the expansion is installed when I tried to use a modified savegame in the non-expanded version. As soon as I tried to move to another area the game would crash to the desktop.
Check my profile for the Glaicas patch