Camps are the s*** man. I used them EXTENSIVELY to plug holes on the map into my lands and leave only one open area into my lands so the AI sends its armies there.

I also use forts to delay the AI's attack, that way I have a few turns to train new troops, transport them, and hire a few mercs before they take my city/s.

I also use forts along borders with neutral/allied factions IN CASE they decide to attack me. Again buying me time to train troops.

Besides, all empires had forts along thier borders to regulate who came in and who came out. They also served as initial defence or as a delaying force until reinforcements arrived.

again, forts are AWESOME!