Siege tower bug

[Wolfstorm]

I was playing RTR-PE1.9 and theres a bug with some siege towers that the AI sends to capture my cities. The units suck at the base and keep trying to enter the siege tower. They only enter it if someone attack them by the back. I dont known if this is a problem with RTR or with RTW 1.5 patch, since is the first time I play with this patch. This problem is similar to the one with the phalanx formation, where some units go far away from the formation. Since this problem is happening with the AI, I had to send troops outside of the city to attack enemy units so that they enter the towers, and if theres a lot of enemy units outside the walls I put my troops into risk.
Do someone have some advice on how to solve this problem?