wasd = move
q,e = turn
TAB = Map (can be turned off)
ESC = menu
RMB+LMB required (except in tablet mode found in the game options)
Additional Info Edit
Windowed works (no resolutions below 1280)
Enable Viacam headmouse works
natively no gamepad, XPadder might work
Just a little warning:
While Grimrock has a control scheme that helps mouse-only gamers, it is still a game that demands quick actions. Casting spells is specially hard as you have to dial in the required symbols which becomes almost impossible in the heat of a fight.
There are puzzles with extremely (!) thight timeframes that cannot be solved using a headmouse along with the on-screen keyboard.
Here’s a link that contains info on how to slow down The Legend of Grimrock in order to gain longer reaction times. I can’t tell if it helps to completely play through the game but it does enough to let you win against quick opponents.