Art style is excellent. Volume sliders got some good use out of them. Everything seems to seamlessly come together, but I found myself wanting to be able to move the source nodes around, or flip blocks instead of rotating them. The first grid size upgrade was a welcome sight, considering. Cross-pipes and x-pipes are the most valuable pieces. Played several times. Only managed to get the second +2 size upgrade once.
The title splash looks good but is very busy. You made want to fade it out a bit or overlay a semi-transparent colour over it to make the title buttons stand out more from it. I think the main issue with the actual gameplay loop is that the amount of health you need to start each round with increases - since enemies always manage to deal damage before you can put your defenses up. This is also why defense/healing is pretty much superior to any other combination, since it increases the health you have to start the next round with.