Ok, so I'm a few levels into Kingdom Rush Vengeance, and there's really not much to worry about, it's definitely still the Kingdom Rush that we know and love.
Main change is that you have a load out of 5 towers (instead of the 8 you always had in the past games, the 4 base ones X the 2 final upgrades), but once you unlock them all, you'll have 11 tiers to choose from (3 more than the past games).
Yes, there are 5 paid towers now, but they mostly seem like tweaks on towers that have been in past games, like one that causes enemies to become zombies (like the Necromancer in the second game), or one that's a mage death ray (like the one in the first game), or a tree one that does damage over time (kinda like the electric one from the first game).
The 11 free towers should offer plenty to play with if you don't want to buy the premium ones (I rarely ever bought premium heroes in past KR games unless there's a good sale).