Very cool! I like the idea of gathering materials and building your defense. The exploding enemy work very well for this mechanic.
However, The lack of audio kinda make this game felt unfinished. for your next project you should considered adding sound effect to the game, even the most basic sounds can make your game feel much better.
For your first game, you've done an amazing job. you've got great idea and good art style. Keep up the good work.