Admittedly confusing, but fun.
This is a tidbit confusing, and it's a bit slow to start off. Trying to get into the hang of things, balancing between metal and gold and energy, before you can properly and sufficiently build an army to hold off those tides of enemies. It takes most of the build-up period for me to get enough income to do so, which is something I personally don't like.
I do like how simple the income system is, and the units and such. It's not overly complicated, but it's simply enough for anyone to understand. And once anyone spends their first level just learning how to balance their income and then properly spending their resources, they can just restart the level over again and know what to do.
A negative point is the building boundary/terrain. There needs to be wider space. If you want players to be able to build how they want, and not how the game forces them to be. It's quite impossible to get through some of the levels, just from how they start off, simply due to the terrain issues. Like in Mission #2, I literally had to build this ONE way, just to get to the metal depot. Which is not something I think is productive to this game.
I still liked it a lot, it's on my favorites list now, and I'm probably going to dedicate a lot of time to it later on in my day.
Something you should consider adding is a tutorial. And also have that tutorial NOT be the first level, and not have the timer keep running while people are reading the instructions, otherwise they set themselves up for imminent destruction.
Another good idea might be different modes. Like a Skirmish/Custom Game mode, and also code in and allow for multiple players (AI that is, not an actual multiplayer mode unless you're confident such a thing can be done.
It's these little bits and details that help make up the game.
Otherwise, you've gotten a 5/5 and an 8/10 from me, and I hope to continue to see great games from you!