Cool approach on defense
From my memory, this is probably one of the first defense games that did away entirely with any resource system to produce things, and made time the only factor. You made it even more interesting by making towers dynamically upgradable. That's an interesting avenue to pursue, but you seem to have stopped after coding for those two ideas and throwing in the basic art and whatnot.
What most peeves me in this game is the inconvenience.
When selecting parts and items for the Star Forge to produce, you have to click multiple times to find what you're looking for. And sometimes when you do, you accidentally click too much, starting the clicking cycle once again. This can be remedied by providing a tray of icons of the parts you can make in a UI somewhere, which would actually justify why this game has a rectangular window.
Speaking of which, the window seems too small when you consider the big blue capital ships interacting with your ring of towers. "Oh, is it going to die soon?" "No idea, I can't see its lifebar sir." It's just claustrophobic.
When the Star Forge spits out tons and tons of items, it gets so cluttered up that you will have a hard time seeing some of your upgrade items and, on a less common basis, the lifebars of your northward turrets. Granted, you could just drag out most things and solve it that way. But this isn't an option for tower construction items and bombs. Sometimes, even the construction of new towers will block out the lifebars of other towers completely. It gets even worse with the tower info windows: target reticules block out the ends of lifebars, and items block your view of the upgrades you added to the towers.
Some of the problems of defense games weren't addressed too well, but it may be universal. It can become quite difficult to replace towers that fall during story mode, as you're not given a whole lot of breathing room (this is acceptable in survival mode), and if you fall behind after getting caught off guard, you fall behind for good. Compound this with the fact that the story mode takes a long time to complete, and you have a formula for grief and "unfun" for those who are less skilled, not to mention it does get boring during the later "levels". Perhaps you can create checkpoints just in case a misfortune falls upon the player.
Overall, you have the makings of a game. Your simple approach is appealing, but at the same time there's not much replay value to be had.
