Quote Originally Posted by Soulflame
You can't tell them to go into towers.. rather you need to move them past them. So if you are on ground level, order your unit to go to onto the wall next to the tower you want.
To capture the gateway, order your unit to go to the wall above the gate (this part is more raised then the rest of the wall). Once they have walked through the tower/are onto the high wall, the tower/gateway is yours.

The other pathfinding bugs I also had... the moving out of the city is particuarly annoying.
I've done plenty of sieges now, so Im aware you cant order men into towers, but rather onto the walls.

------ = wall
xxx = my troops
outside is outside of wall, inside is inside.

outside

wall A.............................wall B
---------| GATEHOUSE | ---------
xxxxxx
inside

Select group X, and only X. Order them onto wall A - they stand there doing nothing. Order them onto wall B, they stand there doing nothing, order them onto the gatehouse, they stand there doing nothing. They run up to the little door at the bottom of the tower and sit there like a bunch of buffons.