Even though mobs aren’t supposed to spawn in a Deep Dark city besides the Warden, there are a couple mobs who can. Firstly, I do believe it needs testing to confirm but I think Illager Captains can spawn under the surface, specifically in Deep Dark biome (Which could cause annoying vibrations and disturb the Warden.) Endermen can teleport inside the city (It’s more of an issue than you think), and they don’t run from the Warden, so that could mess gameplay up. The third mob is the Wandering Trader (Yes I searched the issue) and it might not be an issue if the Trader spawns in the City, but if a Raid Captain and a Wandering Trader spawned in at the same time, the Trader would die, a Sculk Catalyst could form Sculk, and then more vibrations could travel and activate the Warden “accidentally”, thus ruining gameplay.