Allied reinforcements
 

Would like this because sieges are pretty hard now..even on co-op..enemy reinforcements kill so many...and scaling walls with arrows banging into your troops...you're left with almost nothing when you reach the last wall ;\

Two words my friend... Siegeworks:)

It would be awesome to call in another army nearby... or actually have your own cavalry arrive - if you are defending

It'll just be like when Rohan came in to save Gondor during the siege on Minas Tirith :D

^^^ epic gameplay right there