The idea looks nice, and it is impressive what has been achieved with only SDL.

I would recommend an in game tutorial. Although the docs might be clear and thorough usually players do not read them. This is entertainment so usually players want something easy to learn and that can be fun even when only learning how to play. Usually an in game tutorial that teaches all the possible actions one by one (including what input the player has to perform) makes the game experience enjoyable even for complex strategy game with a lot of rules, attacks and counter attacks, like this one.

The pixel art and the music are nice and enjoyable, I would recommend a little bit more refinement on the images to make them a bit clearer or scale down (not sure if this is a good idea though) the resolution and the screen size (probably making it playable in a window? just throwing random ideas around). Experiment with the look of the game with different scales or with images refined a little bit, not sure what the best approach would be please kindly verify and experiment a little.

Well done for this idea, the technology and the art. Just some more work on explaining the input and the rules and it will be even better.

P.S.:

Please add a legend in the start screen: it took a while to me to understand how to start the game. I was pressing 'enter' when start was highlighted.