From the announcement:
You’ve just found a mine filled with energy crystals ! Precisely those you use for your daily magic. The problem here is that you’v just stumbled onto a Hord of monsters. And to get things even worse, the other cities seem to have discovered the existence of that mine as well. You don’t have a choice, it’s them or you, with the monsters in the middle …
In approximately 60 minutes game for 2 to 4 players, or alone with the included solo mode, you will have to build towers to protect the path leading to your city and send back the monsters to wherever they came from (or to your neighbours’ place, it works too, and it’s actually more efficient).