Hello,
I had and encounter as Dehli, built a stone wall with infrantry (with the landmark) and then tried to get a stone tower.
I cancelled the stone tower due to ennemy approaching, but the little gate that allows infantry to get on top of walls stayed.
I think it should be easily reproductible. I will watch out and keep you updated if I encounter this bug again 
Hmm 
that wouldnt be a bad bug imho… getting ontop of walls sometimes when u dont need a tower or gate there or cant afford it, would be decent… but bug 
yeah prob
This could be caused by placing the gate in wrong facing. There is a arrow on top when building the gate which indicates where the gate is facing. If the facing is wrong, the enemy can get on top of the wall. Because the inner side is suppose to get friendly army to get on to the wall.
Me playing as Chinese had same Issue and the enemy AI was also able to go onto my walls (through) the gate, while it was closed.