Interesting point of debate this and one i asked about at CA.

The pilosophy behind the seges is like this:

In reality seiges took weeks. months even years - and from a gameplay perspective there not much joy in watching it take 30 minutes to reload a mangonel or trebuchet and then wait weeks, months, years to assault a breach...

So - that's why the walls go down quickly - its to represent the "breach achieved" and assault of a castle rather than a "full seige". As the player you are representing your "last ditch stand" if you like.

Now for the seige engines:

You GET TO PLACE THESE in a boz anywhere around any castle - during placement the range targeter still works fo you can mouse over any wall or tower to see if its in range etc.

So even against a fortress (ive doen this) its perfectly feasible and works well - heres what i did:

Make up about 8-10 seige engines and put in 8-6 support units to defend them from sallies from the castle and AI.

Once you've breached the walls in the right spots "Withdraw" the seige crews and bring up your reinforcements (assault troops) and then go in for the kill.

Or various combiantions thereof.

Works like a charm... the late end cannons can reach almost front-to-back even on fortresses btw so it really isnt a problem depending on castle design and your assault plan, LOF etc.