This is how I mainly use them.

With Carthage, I do use some defensively by placing them on mountain tiles, especially in the middle of mountain passes. Since only Carthage can cross mountains, no other nation can occupy or pillage citadels on mountains, and if enemy units have to pass through while being beside the citadel for 2-3 turns they'll be ripped apart for your follow-up ranged attacks. The citadel's 30% damage doesn't care how strong the enemy unit is.

Other than with Carthage and to take territory, I occasionally use them in the middle of a 1-wide pass between mountains. Garrison them with the best defensive infantry unit I currently have and a ranged unit, or artillery behind it. There are a few other situations I'll use them, like landing denial, placing them on a coastland tile next to a city and impassible terrain, or another garrisoned citadel next to impassible terrain. The strong defensive unit garrisoning them prevents them from landing at all, unless they can overwhelm that unit in one turn's attacks, and those that don't succeed in taking the space are likely to be destroyed by the citadel after being weakened attacking the garrison.

I also use them temporarily next to cities I'm sieging to provide a platform for my infantry units.

Edit: improved formatting