I do not like the suggestion to prohibit castles on some terrain, but I do like the suggestion of "nocastle"-flag similar to the "nostart" flag!
This would enforce nothing but would open up a lot of possibilities for mapmakers!!!
Especially scenarios with preset castles would benefit form a "nocastle"-province-flag!
I wish this flag existes, so that I can prhibit castle building on the bridges in my own Chandrea map...