I also agree with 10% for neutral creatures and all castles when dividing zones. This would be fair and interesting. 25% is probably too much (especially considering Necropolis, Dungeon, and peasant hiding spots), but 10% is logical and balanced.

This is only for zones without castles. The chance that there will be more than one such zone on the map is small for most templates. Where does this "logical" figure of 10% come from, other than "take 100% and divide it by the number of castles + neutral creatures"? And what about the fact that zones with castles have already occupied most of the map? We've been playing with these "sheds" for a hundred years, but now that Heroes VI+ has come out, it's suddenly too much.