“the threat outside the city would be the Russian army then?”
No, the very large concentrations of UK troops and artillery north and west of the city are the threat, which is why control of the airport is so important. Controlling the Nuclear War bunker system under it is critical, for both attacking the city, and defending it.
The similar mine tunnel system to the west and southeast of the city is why the UK’s could gain no real ground, despite local superiority there, before the Illiovsk debacle.
The real threat are the pogroms from the Russian Taliban it sounds like a real Lord of the Flies going on.