If the game was crashing before you could get to the tower, there is a good chance the terrain load is corrupt, or your computer is right on the edge of running out of resources and the SEA terrain, with any number of custom objects, put it over the top.
For the latter, reduce anti-aliasing (assuming you have some set) and/or uncheck some of the Options->Graphic Detail->Advanced options (big shadow texture sizes are a good one to reduce).
