This has been noted elsewhere. There's a game crash bug when armies are 1,100 in size or some huge number. Do a search. Not sure what the resolution (if any) is. Other than trying to cast ritual spells to whittle his army down (unless it's YOUR army that's too big, then just attack with smaller armies).
EDIT: Here it is:
http://www.shrapnelgames.com/cgi-bin...=000657#000000
Try changing what you do on that turn.
[ January 31, 2004, 15:35: Message edited by: Targa ]