Great game with a well-executed idea! I've seen a couple other "tower defense, but with one tower" idea and I like this one the best because it really leans hard into what's cool about tower defenses (comboing tower types, juggling lots of threats at once, things that bend the rules, etc.) I felt the first few levels worked great as teaching tools, I played 3-4 levels then restarted and got to level 12. Assuming level 12 is the last, that's a perfect length.
Someone else mentioned this but I felt kinda cheated on the last level because taking out the mortar enemy had no effect other than wasting my time. Might be just better to make him invincible to better communicate that you should focus on the actual laning units. Still, mortars were my favorite part of the game because they forced me to micro and made me feel cool for dodging their attacks.
As an incredibly small pedantic ask, I feel like it would be nice for the tower to fire immediately upon being built rather than having a delay, which would reward the player for being able to switch quickly.
There's a ton more potential here (the game is already great!), would love to see this expanded on with more levels, upgrades, tower types, etc.