I use forts as recommended above to block choke points and high speed avenues of advance.

The neat thing about forts, is garrison size to maintain it. So far, I have used as little as 2 guys (not two units but one remanent unit of 2 men). The AI would rather walk around (like Spain/Gaul border putting a fort in the middle pass) than take the time for his 1000 man army to take out the 2 guys "holding down the fort."

I have yet to do this in V1.2, but would be pleased to see the AI attack more often.

Note that a garrisoned fort projects a zone of control (ZOC) stopping movement of enemy armies.

Forts serve to give you early warning of invasion and buy you a couple turns to prepare. Well worth the money to build them (just leave the smallest garrison possible).

Now bridges are different. The concensus above is the way to go for bridges near the border of an enemy until you can advance to take the provence.