Not what i had in mind when i read tower defense.
couldn't upgrade anything.
pretty much spam the three spear man, knight, paladins to the point the enemy's die the second they meet. gets a little boring
the same difficulty most of the way and when you finally finish the game freezes?? could add a endless mode for those trying for highscores.
could add a boss monster every 10 levels.
maybe some castle upgrades maybe fortify. or some sort of oil fire type stuff
or ability to hire some archers maybe a mage
or some aoe abilitys. example slow the horde. or increase strength , maybe lightning bolt anything to make the game a little more active.