[Edi]

Spell modding so that you set all the spells you want to research level 0 and all you don't want to -1 (unresearchable) should do the trick for spells.

The castles, no. For that, you would need Dominions 2, where the AI did not know how to build castles and was therefore always easy to beat. Had nothing but hordes of indies too. That was the #1 complaint about Dom2 for SP players.