Useless ladders ...

[RabidGibbon]

Yep the screenshot does show an OTT example (Although there were some hoplites, archers and peltasts on the other side of the gatehouse). It was just to show the technique, and in that battle it was troops that used ladders that captured the gatehouse, whilst the towers that where closer were still loading up and hadn't even dropped their ramps yet, opening the gates and letting troops in whilst the defenders on the wall were still fighting troops who'd climbed over on other ladders.

So in short (In my opinion only of course), Towers are good against defended portions of the wall, ladders are good for quick assaults on undefended sections of the wall (Which can sometimes decide the battle before the tower using troops have cleared the defended sections of the wall).