Fairly major bug: you can upgrade microbuildings and sell them for HUGE profit. If you upgrade them enough, they'll sell for negative profit (won't even be able to buy the 0 cost upgrades anymore). Taking this to an extreme, I upgraded my buildings so they could do damage in the millions. This results in the screen being completely clouded over with giant yellow numbers and obscures all vision (though I imagine being able to upgrade to such a level was never intended to be possible).
Lots of little tweaks that’d add a lot. Having the camera start completely zoomed out and static while allowing you to zoom in for better control and focus. Nice job with the difficulty/design on the spinning blades and the heavy tank-tread guys; but the walkers were way, way too slow to be even an annoyance and the later enemies were still easy to dodge thanks to only ever firing 1 bullet at a time. I would recommend just showing all weapons/upgrades from the start to give players more customizing options (and more replay value).
Add enemies that can fire homing missiles at you so that it isn't just circling around the area to avoid being hit the entire game (though the spinning blades were nice in making that harder to do). Also actually needing to use the shield every now and then would add an extra layer of depth and strategy (enemies with odd patterns like the spinning blades would make dodging less predictable and give more reason to use shields).
Well made game regardless.
For those that feel they should recognize the music at the very beginning (while the guy is dancing) but can't, it's Crash Man's stage theme at ~25 seconds into the song (Megaman 2).
 "Sentinels: First Wave Beta2"){kind=icon}
 "Mech Defender"){kind=icon}
 "No Time To Explain"){kind=icon}