As you indicated, having a hole in the wall is an advantage since the enemy will only try to enter through that gap rather than climb the walls. If it's not too earlyish and you have Grenadiers, try the following.

Just put a solid line infantry unit across the gap set to defend and mêlée, but only 3 or 4 ranks deep. Then put some Grenadiers right behind them tossing bombs right into the gap. If your line infantry has too many ranks they might take too much damage from the grenades. This worked pretty well for me a couple of times. When about 50 or 60 grenades go off in the middle of the enemy unit they usually route right away, but have more line infantry and Grenadiers ready as you lose men.

Since you are early in the game I'm guessing you don't have mortars or howitzers yet, but if you do--- set them up in the middle of the fort so they can cover any approach.
Also Howitzers, which can lob shells over the walls, will also do some damage as the enemy approaches.

I've only had the occasion to defend a fort about 3 times in playing steady since the game released, but then I'm pretty much always on the attack anyway.

Cheers