I tried this game on a steam free weekend and I'm glad that I did. It's full of fun and customisation; a game that isn't very serious, but is seriously fun.
It's a hoard style defense game, coupled with mining in a dark environment. There are four classes to help you navigate the caverns and each one of them is really useful to the team.
Each mission has multiple difficulty levels called "hazard level" and you can really feel the difference between the levels, allowing for constant progression. The game also allows you to customise your dwarf in tons of different ways both aesthetically and with weapon upgrades.
Overall, it's great fun to play with strangers and even more fun to play with friends. It's well worth a try on a free weekend if you've not played before, and if you're on the fence, I would encourage you to take the risk and buy the game - I don't think you'll regret it.