This is a very addictive tower defense game.  There are a lot of these, but this one focuses on the most fun elements of this genre.

Heroes

Each hero has different specials and abilities that can be upgraded over time as you earn more experience.  They dramatically change how I play and adds variety to the levels as I replay them to earn more stars.  However, they are unlocked over time, and having to level up a hero is somewhat tedious.

Upgrades

As you earn experience for  your heroes or complete a level, you often receive stars and points to upgrade something.  I wish the upgrades don't come in parts, and I don't like the fact the earlier levels still restrict you with limited towers and upgrades.  But I do think they serve a purpose of having players try out all towers and upgrade paths before settling on a few favorites.

Short Building Time

There is a few second of building time for each tower initially, and instantaneous upgrades after that.  This is great, because it means I don't have to manage the build queue (one the mechanics I dislike the most).  It helps to make the game feel focused on purely what towers do and their cost.  Towers cannot be damaged (although they can be disabled), and so player doesn't have to spend time on fixing them.

Enemy Slowing Mechanic

In many tower defense games I have played there is usually a tower that can dramatically slow down the waves of enemies marching towards their goals.  Such tower exists in Kingdom Rush Frontiers also, but their effect isn't as pronounced as other games.  This game does have a type of tower that trains militia (assassins or knights after upgrade) who can stop and fight enemies.  The game also allows player to immediately plop down two fighters anywhere in the map or redirect hero to different choke points to immediately stop enemies.  The units and hero engaging enemies will take damage over time, and with enough numbers enemies will eventually move forward.  Being able to relocate units on the map and create choke points organically is a great design feature.

Everything in Single Screen

Unnecessary to scroll around, therefore not necessary to bind positions to hot keys and overall simpler control scheme.  Some UI elements do come in and out of the screen and so it is possible to accidentally click on them.  Perhaps they should be nailed down or removed altogether if they are not important enough to be visible all the time.

Uniform Design Language

There are many comic-like intermissions and humorous voice-over or texts.  This fits very well with the cartoon-like aesthetics of characters and graphics design.  I imagine it took significant effort to make everything consistent, and the result feels great and immersive.