Under Monarchy I like to take a few strategic mountain, and hilltops. Just fortify a unit there then if he comes under attack I'll send another so he can't be bribed. If The attackers are decent that a few might take out my top defender I'll build the fortress. If I ever see a mountain, or hill with a river through it I fortify a unit there. Eventually he'll be attacked along with the others and this takes the heat off of my nowall city defenders on flatland. When going to republic or democracy I fortify those defenders if I haven't yet.
How about those bad@ss AI forts? 90% of them are on flatland, and the other 2> one on a river one on a hill. All empty of course. They seek out your mountain defenders primary, while NEVER fortifying on one themselves. If they finally bribe your lone mountain defender (that they just splatted 20 units on) the first thing they do is move it off lmao.
Anyway, I usually pillage more forts than I build: after taking over some territory. They're ugly and I don't want barbarians landing in them.