Ironically, this means that Astrum can switch to defensive settings and walk into Unterstrom without a battle. They can then declare war once they have 100% of their troops in place (no stragglers), and I wouldn't be surprised if some funkiness results in a battle without walls (I seem to recall similar funkyness in the past).
If the city is SA, they can send in a priest without worry of arrest, since they aren't currently at war with Avernus. Kicking the lord out would allow for the region to be bought.
It's still technically Swordfell's capital though.