Solo play is completely possible ( I have played all the content several times over, solo )

The game is approx. 1/2 completed so far, with 5 out of 9 areas (called biomes), complete with bosses in each of those biomes, already in the game.

For an early access title,, it is surprisingly "polished" and the content feels like a completed game, with very few bugs to speak of. (doesn't "feel" like it's still in development)