If you are hunting for games like They Are Billions, Age of Darkness: Final Stand is the one to buy first. It runs the same build-by-day, panic-by-night loop, it left early access in January 2025, and it is the only close relative that lets a friend hold the walls with you.

They Are Billions came out of early access in 2019, and the studios chasing it since have each pulled the swarm-defense formula somewhere different. Pick by what you miss most: the horde scale, the zombie setting, or the specific dread of one leaked infected ending an eight-hour run.

1. Age of Darkness: Final Stand Is the Closest Match

Buy this one if you want They Are Billions with better unit control. Age of Darkness runs the same fortify-and-survive rhythm against Death Nights that push thousands of units at your walls at once, and PlaySide Studios shipped co-op with the 1.0 launch in January 2025, which is the one thing the original never had. It sits at $27.99 on Steam.

Three factions with two heroes each give you hero units in the Warcraft 3 sense, so a sharp micro player can plug a breach the towers could not. The trade is tension. There is no infection mechanic here, so a single leaker does not cascade into a dead colony, and anyone who lost a 200-day run to one zombie will notice the safety net.

2. Diplomacy Is Not an Option for Absurd Horde Math

Play this when you want the enemy counter to get stupid. Door 407 left early access on October 4, 2024 at $29.99, and the pitch is a medieval lord defending a walled town against tens of thousands of peasants arriving in one unbroken line.

Wave design is the reason to own it. Time-stop and meteor spells sound like cheating until a siege tower column reaches your gate and you are down to one spell charge. The weak spot is the campaign, which mostly teaches systems, so survival and sandbox are where the game actually lives. If you want the same siege pressure without the fantasy magic, the classic boom-and-rush picks handle that fight straight.

3. Infection Free Zone Puts the Horde on Your Actual Street

This is the pick when the zombie setting is what you came for. Jutsu Games builds the map from real-world data, so you can drop your survivor base on your own neighborhood. Days go to scavenging and repair, nights go to holding the perimeter, and it runs $19.99.

Be clear-eyed about the state of it. The game is still tagged Early Access after launching that way in April 2024, and the PS5 and Xbox Series versions that landed in August 2026 shipped on that same build. Squads still snag on real street geometry, and night defense is tactically thinner than the wall puzzles you came from.

4. The Riftbreaker for Defense Without the Undead

Take this one if you love the base-defense loop and are finished with zombies and castles. EXOR Studios dropped you into a mech on an alien planet back in 2021, and survival mode is a long fight to keep walls, turrets, and power grids alive while the local wildlife escalates every night.

The catch is that you fight personally, so half of it plays like a twin-stick shooter rather than an RTS. There is no co-op, and there is no map-wide creep punishing you for expanding greedily. It scratches the defense itch and leaves the colony-management itch mostly alone.

Multiplayer and Mobile Options Are Thinner Than You Want

Ask for games like They Are Billions with multiplayer and the honest answer is one title long. Age of Darkness co-op is the only place here where two people defend one settlement, and every other game on this page is single player by design.

Mobile games like They Are Billions barely exist as a category. Rebuild 3: Gangs of Deadsville on iOS and Android is the closest honest recommendation, and it swaps real-time walls for turn-based survivor management across a city grid. If a phone is your main machine, these offline mobile picks will treat you better than waiting on a port that is not coming. If what you actually miss is the colony rather than the siege, story-driven colony sims point the other way.

Which One to Install Tonight

Go with Age of Darkness if you want the closest fit or you have someone to play with. Take Diplomacy Is Not an Option when you want scale over subtlety, Infection Free Zone when setting beats polish and you can live with early access rough edges, and The Riftbreaker when you would rather shoot back than watch a wall. Strategygame.org keeps the wider genre breakdowns in the real-time strategy guides if you want to browse before you spend.

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