Many games have different levels, usually the game gets harder as you go through the levels.

We're going add levels to our simple game.

Each level will have a different background and we'll use the background to add difficulty to the game.

Pick a colour as a 'wall' colour, say, red.

Now, edit the original background and clear it all to black and name it level1

Copy that to make level2, add some walls etc.
Then level3, which is harder etc etc.