I'm really loving the variant magic rules in the Castles & Crusades Castle Keepers Guide.
Also the alternate math for the Siege Engine is cool. I've noticed that there are different ways to arrive at the same result myself and this makes it really clear and shows the advantages of each approach. I prefer 15/+3/-3 - elegant. For more on the siege engine and how it works take a look at the quick-start rules here.