In the current state of the game, as far as fort wall defense is considered, the following holds:

1. the wooden fort is the BEST for wall defense; cannons are deadly so is rifle fire from the walls.

2. the gun fort is the second best, but worse than the wood fort; player units seem to start to fire when the enemy is at much closer range than in the case of the wood fort; accuracy is worse.

3. the star fort is the WORST for wall defense; player's units fire cannons only at point blank range with almost zero efficiency; rifle fire from the high walls almost always misses the enemy.

So, this progression leads me to suspect, the walls as coded into the game now are too high for the current rifle range, which is pretty short...

The vertical distance of the units from the top of those huge skyscraper walls almost matches the usual 'aiming distance' on flat ground.