Ultimate Zombie Defence is currently free to claim on Steam, but this is only available until February 11 2024 at 6pm UTC.
From the Ultimate Zombie Defence Steam Page:
Ultimate Zombie Defense is a Top-down Co-op Survival Horror set in the suburbs of a once quiet city, in the heart of continental Europe. The city and the surrounding 250km landmass have been sealed off in an attempt to curtail the outbreak, although sadly all attempts have failed, until now.
There is no official Steam Deck Compatibility Status at the time of writing this article and the game did error for us on launch under the stock proton, however switching the compatibility to Proton Experimental got us up and running on the Steam Deck.
You will need to map a few buttons to the Standard keyboard and mouse layout though to play properly, the B, P and Enter Keys are required to enter build mode, the gun shop and to progress rounds.
You can get Ultimate Zombie Defence on the Steam Store here