I really like the idea of turn- and grid-based combat! I'd like to focus on a high-level design concern for my feedback. It seems like the order in which people get to move will confer in inordinate advantage. My recommendation going forward is to add gameplay which allows for defensive or reactive play, i.e., using a turn to place an unseen counter, which anticipates a later player's attack and deals damage to the attacker instead.
Otherwise, I thought it was pretty fun to play!