I'll often build extra ladders, and have a unit or two with a ladder far away from my main attack force. In almost all cases, this will pin 2-5 defensive units at that part of the wall and away from my main attack.

As soon as the main attack starts to succeed, those units storm the walls, and if there's archers in the bunch, are able to fire into the backs of the troops headed to reinforce the main breach.

If you have a situation where a spy has opened the gate for you, it's even more helpful, as you can take the wall uncontested while all troops rush to guard the gates, and cause chaos within the town as the defenders are trying to get their act together.

As an aside, if a spy opens the gate, is it all gates, or just the one you are facing? If the former, you could send a small unit of light cavalry to harrass the defenders from behind.

Gdek