In most places the author can't enforce a copyright on core game mechanics. No one is alowed to put a stamp on "the first person control scheme." Art, sound, story, setting, levels, all those are the intellectual property of the game.
If you're asking if you can make a top down game where you run around and kill things then yes, go ahead. If Hammerwatch inspires you to make something new that's great. Hammerwatch certainly isn't the first, nor will it be the last, game to let players press keys to run around slaying monsters.
It does happen to be a pretty fun one though.