An upcoming base-building RTS from the developer of Moonlighter and The Mageseeker has a demo on Steam showing off the versatility of its building system and how potentially exciting an RTS it's shaping up to be.
Fans of base defense games like They Are Billions should take note here, as it's a blend of that style of RTS base building and a more freeform defense-crafting that allows a lot of creativity. Buildings like houses and gathering stockpiles are fixed size objects, ala a traditional RTS base, and the units work just as you expect. But the blocks of your walls, towers, and traps are placed one by one with a remarkable degree of flexibility.
You can make spiraling towers, defensive bastions, and more to maximize the strengths of your different unit types. Bowmen, for example, are strongest from a high place, while soldiers who toss explosive spiked balls are best up close where they can guarantee accuracy. Funneling the overwhelming hordes into crafted kill zones takes not just architectural skill, but an understanding of how to exploit the available terrain.
What sets Cataclismo apart is that it definitely has a story and some real worldbuilding work put into it. Some kind of magical cataclysm has left this fantasy world as a very dark fantasy, with the world shrouded in oxygen-less poison mist that's filled with horrors who emerge to attack each night. Human settlements must then use pumps to filter their own oxygen supplies, and people have to wear breathing masks to get from place to place. The story seems to center around a prophet with magical abilities who will have to lead humankind beyond the safety of their final city.
The demo out now for Cataclismo explains the basics of how to play in a tutorial and gives you a few nights of base defense to play with. What's most striking is how resources work: They're a constant flow, but capped in total storage. You're therefore really incentivized to spend them as they come in so that you can make as large and effective a defense as possible—only limited by how much time you have before the next sunset.