I wanted to share my experiences with a card I tried recently that has been awesome:
Yes it's expensive, yes it mana intensive, but wow is it fun and synergistic!
The reason I wanted to add this was to include black into my Izzet artifact and spell decks. In my cube, Izzet plays both of these archetypes to some success and this seemed like the perfect mix to push people into black.
To maximize it's potency, you need a good mix of cheap spells and ideally some card filtering to clear the top of your deck should you hit a brick. This in my eyes is just good deck building anyways, but this encourages you to do it more explicitly. Here are a few (high powered) deck lists to illustrate
In this first list we have a good mix of spells and artifacts. The mana rocks and Daretti make sure that you can cast your Citadel ahead of the curve. What makes the deck really fun and powerful though is that it is able to use both parts of the Citadel: the card advantage and the sacrifice! With cards like
Filigree Familiar and
Young Pyromancer providing road blocks and sacrifice fodder, you are able to actually activate the Citadel!
The fun doesn't stop here though.
Pyromancer's Goggles is another important part of the artifact/spells crossover that allow you to play Welder and Daretti in other roles than fatty cheats.
Yawgmoth's Will also plays nicely in the cheap cantrips/spell decks as a form of redundancy.
UB control
https://cubecobra.com/cube/deck/5e405d393611fb6916ab333e
Here is a UB artifact control deck that focuses on spell/artifact velocity with Citadel,
Saheeli, Sublime Artificer,
Urza, High Lord Artificer,
Thing in the Ice and
Rise from the Tides. I would have liked one or two more spells in that list, but it is still fine. Here we have a third piece of the artifact/spell crossover
Torrential Gearhulk.
BW drain
https://cubecobra.com/cube/deck/5e5e97a5a0b97a386ec638cb
Basically an aristocrat/aggro deck that can use the Citadel as card advantage and/or a finisher combined with all the other drain effects (which conveniently give you more life for more cards). Card velocity is once again the name of the game with
Monastery Mentor,
Blind Obedience and
Seeker of the Way.
All these decks exemplify how (I feel) is best to maximize Bolas's Citadel: card velocity and cross support in other colors.
Card velocity: Chromatic Star,
Faithless Looting,
Preordain,
Night's Whisper and their ilk are so important for the decks to function. They are also great at smoothing draws and allow you to double spell often, which I enjoy as it feels like there are more decisions in a game. Some (like the Star) are super important as they are colorless (giving access to cantrips to white for example) and have added bonus (like being sacrifice fodder). They won't do much without support, but provide enough cards and they become staples in a format.
Cross support: So (hopefully) you are considering the Citadel! Well alone it won't do much. But if you support artifact and spells deck in Izzet, then you probably have most of the pieces for it. Here is a quick list of what I deem interesting or essential support (essentially card velocity payoffs, artifact payoffs or sacrifice payoffs).
In multiplayer I also play extort cards as card velocity support (
Syndic of Tithes,
Basilica Screecher,...).
So there it is, a wall of text for a single card, but I've had such good experiences with it I wanted to share. Once again, my format is slower (multiplayer) and more high powered than most of the cubes here, but maybe you can make it work for you too!