Here's an example of a simple game implemented in a single HTML file using JavaScript. Please note that the code below is just a starting point, and you can enhance and customize it further to fit your requirements.
In this game, you control a blue square representing the player and need to defeat two red squares representing enemies. You can move the player using the arrow keys on your smartphone. The enemies move randomly within the game container. Additionally, you can build walls by tapping on the game container, and each wall costs 3 units. You start with 10 walls, and the number of remaining walls is logged to the console.
Feel free to modify and improve this code to make it a more complete and enjoyable game!