I love TD games, surprised there aren't more at LD! It's a decent prototype and you should be proud if that was your goal.
Some thoughts that came to my mind: - you have 2 types of towers (plus shield generator), but 5 different kinds of resources. Usually, I'd suggest the other way around, so the player might build some towers that require more of a specific resource for example - I don't know what upgrades do. I know, things do more damage, looks like have a bigger range, but as a player, the game doesn't tell me in any way whether it's worth building another tower, or upgrading an existing one. - a bit related to the previous point - there is not much "strategy" here - if I knew that, for example, upgrade costs 3x more than building stuff, but gives me 5x the fire power, I could choose between a bit more danger now, with saving money for upgrade, or investing in new towers now, which might hurt me in future rounds, but helps me now - would be cool to see some more diversity with the towers, so I can choose between big damage, or slowing enemies down etc