Very sleek tower game! It gets addictive very quickly! Your tooltip setup/system is rather helpful and rare to see. I'm eased into the gameplay so smoothly without the slowness of usual tutorials that take forever.
One feature I recommend is a Sudden Death / Critical Health bonus. When the King/Queen you protect is at 10% health or lower, your unit team size increases by one just for that round---only occurs once and also goes away when health recovers above 10%. Because you're very likely to lose at such low health, I think it'd give the player a motivational "Hail Mary" hopeful chance of making a way out of this unlikely situation.