Horning in on your territory

[Bigby]

Another faction enters the area and begins setting up a base. Not a seige. If they are friendly, this might help you out if they get raided, but the areas resources would be more divided. Alternatively you could intervene and fight off the raid attacking them.

If they are hostile they will continuously gain members arriving from their faction and try to crush you. They will of course setup defenses and a proper base in quick fashion.

[billycop32]

I would call the hostile version a siege still,just a different method of going about it.
+1 good idea for late game epic fights.