This is out of early access. It looks like it uses free to play as a type of extended demo, and then you can buy the rest of it later. I’ve just played through the tutorial area. Game loop is gather resources -> upgrade stuff (equipment, defenses, etc) -> defend base to open more upgrades.
Resources are gathered in the world and respawn. Plants, minerals, mobs all have stuff you need.
Base defense uses towers and you augment them by running around helping. Not sure if it can be done with towers only. Combat is simple, but I did run into a creature that required me to dodge a bit. You can drop down towers while in the world gathering resources to help with more difficult encounters.
I need to give it more time before I know if I like it or not. But since the beginning is free, I suppose that only costs time. :)