I see that instanced city defense is being considered.
"
The majority (~80%) of content in Ashes of Creation is open-world but there may be instanced battles within castle and node sieges"
https://ashesofcreation.wiki/Node_sieges
Why not make them happen in open-world?
Cannot be a performance issue because players are present on the server anyway. Can the server not handle the event?
For immersion, it would be better to see the siege happening, as you approach the city. And then decide what to do.
Finding a way to enter the city and join the defenders could be a challenge in itself.
Or a big party when it arrives to the siege location, could help from outside too.