Game core mechanics are executed fantastically and everything works well. The main element missing is the incentive for the player to move anywhere since there is no in game benefit for the player of doing that.
Otherwise to spice up the overall gameplay designing a goal like player collects parts to fix his ship to escape the aliens would give the incentive for the player to move around the map to explore and collect items to achieve the given goal.
Overall, the gameplay is solid and fixed with an understandable amount of complexity in your implemented game mechanics.