7 – Second Earth

The developing team that worked on games like Terra Nil and Broforce, Free Lives, has worked on a very early prototype of a horde-defense strategy game called Second Earth in 2019. To this very day, people will go to the itch.io page and ask about its status, and lament how it was never turned into a fully-fledged title, because it had a ton of potential. Fortunately, for us, this impressive horde strategy game is still available for free, and you can try it out right now.

6 – Conan Unconquered

Conan Unconquered is a pure, unapologetic base defense RTS, and one of the closest spiritual cousins to They Are Billions on this list. Set in the brutal world of Conan the Barbarian, the game tasks players with building fortified settlements capable of withstanding massive enemy onslaughts that grow stronger with each passing wave. The defining feature here is scale. Enemy armies can be quite something, and if your walls, towers, and kill zones aren’t properly designed, they will simply be overwhelmed. The game encourages layered defenses, redundancy, and clever use of terrain, as any great game of this nature should. Unlike most games on this list, Conan Unconquered allows players to team up in co-op mode.

5 – 9 Kings

9 Kings instantly became one of the best games of 2025. In my review, I called it the perfect and most fun time-waster, because this deceptively simple-looking horde survival strategy game and auto-battler hides a shocking amount of depth beneath its minimalist, pixel presentation. At its core, it’s a kingdom defense RTS/strategy hybrid where players must protect their realm against increasingly overwhelming enemy forces, using limited space, smart placement of units and buildings on a limited grid, and building their deck considering long-term strategies and possible unit-to-building combos.

Unlike traditional RTS games that rely on large maps and constant expansion, 9 Kings thrives on constraint. You’re working with a compact kingdom layout, which forces every building, unit, and upgrade choice to matter, and replacing the wrong building on the wrong turn can instantly spell doom. Enemy waves escalate rapidly, and poor early decisions can easily snowball into an unwinnable situation later on. I loved it, and I still play it quite often.

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