As things stand Citadels are a joke, it has gotten to the point that i go out of my way to let the scylla retreat because citadels are easier then lairs.
I was thinking about ways to make them harder and thought you could add a room with a door (like in the final mission) and the door wont open until all non eggs non towers outside are killed. The scylla would be inside surrounded by eggs (her hatchery). Each turn she heals for a X amount. (Also grenades cant go over the wall)
Once the door opens she could let out a scream which would make all the eggs in her room prepare and hatch the next turn.
This would make citadels unique as well as make them not a turn 1 finish every time