Tried that, got a bunch of my guys killed like that. The reason in this situation was because the towers were coming in at an angle, and I didn't expect such a wide field of fire.

Now I keep my guys below the walls, and rush them up when the towers do their final approach (when the machineguns ... er ... ballistae stop firing). I've also got a no-large/epic walls policy now, as there is really no reason to build them except to increase siege hold-out times.

That is one option, removing large/epic walls altogether. Maybe I'll give that a try.