What is Citadel Zero?
Imagine if you smashed a tower defense game and a deckbuilder together, then sprinkled some gacha mechanics on top. That's Citadel Zero, a wild ride where you defend the last alien citadel using bio-engineered defenses.
Gameplay
You're defending the last bastion on a dying alien world, but you aren't just buying towers from a menu. You're summoning bio-engineered defenses via a gacha system and forging a custom loadout to take into battle. It's not like Slay the Spire where you draft cards mid-run; instead, the deck-building happens in your meta-progression. You spend your time experimenting with which units to bring, hunting for those broken combos where one tower buffs another into oblivion to hold the line.
You'll split your time between three modes: the story-driven Chronicle, 1v1 PvP Ascension, and the roguelite Contracts mode. Contracts is where the run variety really shines and where you'll feel the most pressure to optimize your loadout. The loop is tight and addictive, though it's definitely more collection-focused than your average indie deckbuilder.
Since it's Free to Play, the gacha elements are the big catch. Depending on your tolerance for RNG, summoning new towers is either a great dopamine hit or a tedious grind. You'll also run into some rough edges; the UI gets pretty cluttered once the screen fills up with enemies, which can make the chaos feel a bit overwhelming. Still, the synergy between your chosen units keeps you coming back to see what actually works.
Key Features
- Bio-engineered tower summoning
- Roguelite Contracts mode
- 1v1 PvP Ascension
- Custom loadout forging
Screenshots
Verdict
Grab this if you love tower defense with a heavy deck-building twist and don't mind gacha mechanics. Skip it if you hate RNG-based collection or cluttered UIs. 8/10.
Play for Free →* Price at time of publication (checked 18.07.2026). Prices may vary by region and are subject to change. This is an editorial review, not a commercial offer.