The Game 10 was one of three different games in the same/similar case from the early 1980s and shared the  module with the Game-20.
A similar game to the Armitron Piano Game watch, this is a horizontal shooter where you are being fired on from one of three positions and must move your ship to fire back. Points are scored based on where the incoming missile is when you hit it (you get the most points for hitting the ship before it has fired).
Given how little space there is for the game, it’s surprisingly playable. The idea must have appealed to the Casio designers too, because the follow-up game (Game-30/301) was essentially the same thing but switched around to a vertical shooter with a submarine theme.
The third game of the series (Game-40/401) was very different.
There’s also the usual time/date/stopwatch too and, best of all for adults who didn’t necessarily want to be seen to be wearing a kids watch, it didn’t look like a game watch (no fancy colours or front buttons).
Prices have skyrocketed in recent years so if you find one of these watches, consider it a good investment.