The developer of zombie plague RTS They Are Billions has released an official level editor that will let you build your own survival scenarios—and play through other custom creations via the Steam Workshop.
After you subscribe to a level in the Steam Workshop, you'll be able to play by hitting 'custom levels' when you launch a new game. The level editor has been in testing for a while and when it launched on Thursday more that 80 player-made scenarios were ready to go. Another 40 have been added since—the levels include a full episodic campaign called the Emperor Series and lots of ultra-hard challenges.
They also include a level created by developer Numantian Games called The 50 Days Challenge. As the name suggests, you have 50 days to build up your colony before the final swarm of zombies arrives.
As for the editor itself, you can access it in-game (hit 'more' and then 'launch editor') and it'll let you create a map from scratch or edit a randomly generated one. You can add custom events and scripts to help build a narrative for your level, and it supports multiple languages. You can find an official guide for it here and if you're stuck on anything then look in the comments on that page, where the developers are helping to troubleshoot problems. You can visit the forums here.
In a Steam post announcing the editor, Numantian also updated fans on the long-awaited campaign mode for the Early Access game. Now that the level editor has launched, the team said they will "focus on finishing the campaign mode", and are aiming for a spring 2019 launch.