Open the shop with the Close/Open button to the right, and click on a tower to buy it. Instead of shooting at the bloons themselves, you'll have to rotate them with A / Right arrow and D / Left arrow. you can also use W, S, Up arrow, and Down arrow to rotate the towers exactly 45 degrees.
Use this turbowarp link to remove the clone timit. Turbowarp: https://turbowarp.org/1212735574?clones=Infinity Thanks to @honeyinjanuary_x for the tower sprites.
