I’ve played They Are Billions quite a bit. It’s both RTS and tower defense. The base building stuff is pretty standard RTS - placing farms, houses, barracks, and so forth. But instead of another “player” (AI or real) building their own base, your opponent is a zombie horde. The map is filled with zombies that will attack your scouts if they get too close, but also can wander into your settlement if you’re not protecting your borders. This can quickly lead to disaster, as (in proper zombie fashion) every time a zombie kills one of your workers, it becomes a zombie. One lone zombie getting into your housing when you’re not paying attention can bring your run to a quick end.
On top of the map full of random zombies, 3 - 4 times a game a zombie horde shows up, with a giant horde at the very end as the win condition for the scenario. So you need to balance building up your settlement, exploring the map, and preparing for the next horde, while also making sure the random shamblers don’t wander in.
It’s a really solid game loop. A lot of hair-pulling when you miss a small gap in your walls, and your colony gets wiped out, but it pays off when those Tesla coils discharge and disintegrate two hundred zombies that were clawing at your walls.
I highly recommend it.