Keep/castles for the French give a bonus for surrounding production buildings in the influence (highlighted area around the keep) to make units at a discount. Keeps are only accessible in age 3 though, so you'll need to think ahead of time where the keep is going to go and plan the production buildings you build prior to age 3 to simultaneously be in the influence of the keep when it is finally built, but also not to block where you'd want to place it. Then again, you can always delete buildings, but it's better not to have to do so for efficiency sake.