I've played this game quite a bit now, and I definitely have a few complaints, but overall I think it's pretty good. Reminds me of the prime days of Super Auto Pets before it was filled with microtransactions.
In terms of issues, some strategies are so much blatantly stronger than others that it feels necessary to win. Specifically, you NEED to have at least one super-high-attack unit for the king at the end, which I usually use the sword skeleton that doubles its attack for. Summoning is incredibly strong, just like in Super Auto Pets, though I think it's countered well enough. Sniper builds (aka the archer, kobold, and stones) feel very underpowered, being immediately countered by the drunk human and later by the shielded knights. The kobold does solve this problem with its incredibly powerful level 3, but since it's a tier 4, you pretty much are never going to get that.
TLDR; Buff some builds, maybe nerf king health scaling to make sword skeleton not required. Otherwise, great game.