I've had Amsterdam's Advanced Fortifications fix themselves between two AI attacks in the same turn yet had the same class of fortifications in India stay damaged for turn after turn...

I've used troops to deploy cheval-de-frise just inside the breaches or light infantry to deploy stakes just outside the breach with some success - wiping out the attacking cavalry just leaves the infantry to be dealt with...

Line infantry about 50 yards either side of the breach and garrisoned in fort buildings with arty in support firing solid & canister through the gap is also good.

I've had good results with two batteries, one firing canister and the other solid shot straight through the plug of densely packed attackers. Kills so many attackers that some units immediately break and try to turn around to flee, plugging the breach for even longer and allowing further shots.

Does wonders for the artillery crews XPs...

Cheers,
Dreadnought2