Score: 147 - Little issue: if the enemys are Faster than the projectiles they can outrun the towers - would suggested a lower firerate but faster projectiles to balance this a little better.
the Game is solid - the mechanik with the portal was interssting but I missed one stone because it was below the UI window - maybe you set it to the bottom or to the corner?
it would be cool to palce towers repeatedly if you have enought stone - but thats a "would be nice"
Well done! I had my fun :D