This game is not without flaws, and your enjoyment of it can depend greatly on how you like it's mechanics. Most crucial among those is a progression system that allows you to level up your towers between missions through a collectible card system. You can buy additional cards as well as boosters with ingame currency (no microtransactions whatsoever in the steam version) and use those to evolve your towers, unlocking the ability to upgrade them during missions, or infuse them, making them more effective. Different tower cards also have different rarities, with some being straight upgrades from their common versions. This leads to the first major flaw with the game, as the card rewards for side-missions are random, leaving you hoping for a specific tower to drop because at some point in the game, about 10-12 story missions in, you will hit a brick wall of difficulty. Once you get the next 'tier' of basic towers properly evolved and infused it becomes easier again, but getting these towers is mostly a case of farming sidemissions in hopes that RNG will bless you with the right rewards. I feel the upgrade system worked well inherently, but adding tiers to the towers completely broke it.
Difficulty tends to be all over the place in general. For example, finishing the normal mode of the game unlocks a heroic mode and two extra long survival missions. But with the towers required to beat the last level I had zero difficulty in the survival missions, often mashing the rush button to send in more gits because my towers ground everything to paste from start to finish. The heroic modes also felt hugely dissapointing, as I was able to blitz all of them with next to no effort up to the last level, which I overcame with two retry's and a bit of tweaking the tower placements.
The missions themselves however, are well-built for the most part, with the only hiccup being the randomized levels you get for your sidemissions. The game essentialy re-uses previously finished levels and adds random buffs to some tiles and inpassable rocks to the other. This meant you couldn't rely on a single layout and had to adapt, which greatly increased their replay value. Admittedly it could benefit from removing the first few levels from the map pool, since they're so small you run out of things to build or upgrade halfway through the harder sidemissions and can only sit and cast the odd spell, waiting for the waves to inevitably overrun your defense.
The towers themselves start out rather bland with the basic all-round tower, the slow tower and the anti-air tower, but later tiers add some much needed variety, with the highest tier offering some interesting options to mix and match into all new ways to make the Touched's lives impossible.
The Touched is the generic enemy faction in this game and consist of a collection of fantasy villains like kobolds, ogres and trolls. The only remarkable enemy is the pink floating marshmallow... thing that occasionally teleports forward and suddenly sneaks past your defenses. No really, you'll know it when you see it... and then you'll rage.
The game has a story and a linear set of missions to follow along with that, but it's cringeworthy at best. There's no major twists, even if the game tries very hard to set one up for you. The voice-acting is mostly terrible, with a special place in hell for the short snippets played during the missions.
TLDR: the gameplay is good, but beware that the difficulty can spike wildly between missions and most of the times the only solution is to farm sidemissions. Also there's a story, but who plays tower defense games for the story?