It is rumored that it will be free-to-play starting some time in 2018, though the paid version will release on the 25th of July. And if you pre-order now, you'll be able to start playing on the 21st of July. Needless to say, I've preordered already
So what kind of game is it? It's a combination of a lot of things ...
Builder — Collect resources and build your fortress with them
Open World — The world is, like Minecraft, procedurally generated. There is no information currently about how large or if it will be instanced like Minecraft or a shared world amongst all players or somewhere in-between.
Tower Defense — Build automated defenses to protect your fortress against the zombie hordes. But like Orcs Must Die, you take an active part in defending your fortress with hand-held weapons.
RPG — Character classes, progression, equipment, loot, etc