Circus Electrique
https://store.epicgames.com/en-US/p/circus-electrique
Circus Electrique is part story-driven RPG, part tactics, part circus management, and completely enthralling. When everyday Londoners mysteriously turn into vicious killers, only the show’s talented performers possess the skills necessary to save the city.
Machinika: Museum
https://store.steampowered.com/app/1507190/Machinika_Museum/
Welcome to the Museum! Machinika Museum is a mystery game where you discover beautiful and mysterious machines of extra-terrestrial origins in an atmospheric galaxy-spanning 3D puzzle game. Will you be able to understand their stories and discover what happened to their previous owner?
MechDefender - Tower Defense
https://freebies.indiegala.com/mechdefender-tower-defense
A unique hybrid between a top-down shooter and a tower defense, MechDefender allows players to pilot the titular Mechs in a heroic defense of their homeland from endless zombie hordes. Wiping out the alien undead will require clever tactics and powerful weaponry. You will build, upgrade and advance your skills and your base of operations as you fight in this Sci-Fi uphill battle!
--BONUS--
Marathon, Marathon 2 and Marathon Infinity
https://alephone.lhowon.org
Aleph One is the open source continuation of Bungie™’s Marathon® 2 game engine. Available for macOS, Windows, and Linux, Aleph One supports Marathon, Marathon 2 and Marathon Infinity natively. Many third party scenarios and net maps are also available.