The totally unprepared city defense troops of Ben City were bewildered by the sudden attack of the elite Tang Army. They had no idea why the Tang Army appeared in the city, nor could they organize an effective defense at all.
Urban warfare also requires organization; if it's just running around like guerrillas, then a troop without logistical support can last only a few hours at most.
Modern urban warfare demands substantial supplies, so city combat like that in Stalingrad requires a stable "rear".
In other words, only when the front line can be stabilized within an acceptable range, can urban warfare continue.
The dwarf officers within Ben City actually have no confidence in maintaining the front line. They can't even locate all their troops: two infantry divisions are deployed in Ben City, but these divisions are light infantry, with average equipment levels and combat effectiveness.
