3 Gamekillers
Otherwise a good game, but the things I hate about it would be the way you have to click around to get a tower on the field. Most tower defense games have you click to build a tower once, then the build time would start as soon as you place it. I really hate having to click, wait, click, place for every single tower I create. That's really annoying. I don't see why you didn't just do the regular thing most TDG's do when it came to placing towers.
Another problem was the randomness of the drops. Couldn't you have made the system either give a certain item from a certain enemy, or even just have a simple money-based system in place? If you were to make it so only certain enemies drop certain items, it'd add a new dimension to the game. It'd make it so you'd have to pick which items to use wisely to last until the next wave of a certain enemy type came back. Sadly, all you can do is rely on luck to get the parts you need.
The final problem: Upgrading towers. Instead of making it so we have to click around even more to upgrade towers, why not include an auto-upgrade option? That way, we can pick which attribute we want it to upgrade the next time it gets xp. Better yet, why not just make the towers upgradeable with parts instead of xp? And it wouldn't hurt to make the towers look a little different each time they're upgraded.
One more thing: The music at the menu was incredibly annoying. And there was no music during the game. Wouldn't hurt to have something playing in the background.
All in all, an enjoyable game if it was more user-friendly. Right now it's just annoying to play. Would've been great if it weren't for it's flaws.