I was playing campaign mode, and it says that you don't have to pay the troops' upkeep if they're garrisoned. I'd really like the money, but the problem is, I can't figure out how to do this. I'm looking for some button, but I just don't see it. A picture would be helpful, but not necessary.
Also, is there some catch to this? Do you need some building, can only a certain number of units be garrisoned, can you not move them anymore afterwards, etc.?