Something else that doesn't appear to be covered in the rules (apologies if I've missed it) is what happens when an army arrives to relieve a siege?
Do both the relieving army and the besieged force combine their strength against the besiegers? This is the normal practice in most other games and the one I would probably opt for. It assumes the garrison makes a sortie at the same time. I would imagine it would cost an additional AP to do this. So the releiving army army has the option whether to use the garrison or not.