FXR
Explorer
I was thinking more about a raid. The not-vikings do not want to occupy or conquer the city. They want to sack it, not so much for the riches they will plunder (but it's a nice bonus) but more to assert their independance over elves. As I wrote earlier, the elves see the not-viking holdings as client states or vassals, and the not-viking are sick of it.So you've throw in the term 'raid' (a quick in & out attack for loot), and 'seige' (an operation to reduced a fortified position by attrition and starvation coupled with extensive engineering operations which will takes weeks, and up to years).
Which is it?
Because of the influence of occult third-parties, no side is willing to negociate, at least for now. The arrival of the giants might however change things.