Duel Princess
Not sure I would call this tower defence, for me that's more along the lines of Defense Grid, Arknights, etc. In this, you have your base on one side, the enemy base on the other and units just move in a straight line, you can't place defences, block lanes or anything like what I normally associate with TD games.
That said, it was still pretty fun, the main strategy element comes from unit synergy, skills and balancing deployment costs. You have three classes, the game gives them specific names, but I find it easier to go with the Fate approach, so you have sabers, archers and lancers.
Sabers beat archers, archers beat lancers and lancers beat sabers.
Within those classes however, you have a pretty decent variety of units, and rarity levels, from common to legendary, the more rare the unit, the higher the deployment cost, thought hey usually have some neat skill or effect to make up for it, including archers that can attack the enemy base directly, lancers that restore mana when defeated, sabers that have shields protecting them, all that fun stuff. You start with a simple deck of units and can get more either by encountering a shop in the map and purchasing more units (assuming you have enough gold), or by completing battles or story events. Running out of cards and losing a battle this way is, thankfully, not a thing here, but having more units to choose form is always a good thing, there's also an overdrive mechanic, where you can fuse two copies of the same unit, increasing their power and HP, pretty useful when needing a strong start or to counter stronger units, specially because deployment cost doesn't increase from this. Further, each princess has their own unique units obtained via story events. Once you've completed a princess' story, their units are unlocked for purchase in the store.
Each princess has their own skills too, with Karla IMO getting the short end of the stick, her skill just allows her to draw more cards, when other princesses have skills that poison enemy units, increase attack power and speed, grant invincibility, etc, Karla's skill just feels kinda useless. Specially as there's no increase in mana generation to complement the extra cards.
There's player level and princess level, princess level increases the HP of your base as well as increasing the level of their skill, you gain levels by completing stages and story events, up to a maximum of 5, the increase in level is pretty noticeable when it comes to skills, at lvl 5 Dorothea's skill can almost wipe out the enemy base in one cast. Player level affects the level of your units, maxing out at 20, each princess story has a recommended level, ending with Zena at lvl 18. Going back to the first area with a max level army was pretty fun, just a couple of units can handle every fight. Princess level applies to each princess individually, so you have to go through their respective stories and events in order to max their skills, while player level carries through all campaigns, even if you lose a battle and choose to restart, you still get some XP towards your player level, that takes some of the frustration from the early stages.
Speaking of princesses, there's quite the cast, the tsundere, the lonely princess that wants to make friends, the chuuni, the whole gang is here, still, they still stand on their own beyond the archetypes they're based on, and their interactions are pretty funny and wholesome (for the most part lol). Each of them also has their own unique design related to their kingdom, with Iris, the steampunk princess, being my favourite. \m/ \m/
The story, similarly, is pretty funny and wholesome, even though the whole thing is "gather the magic crests to defeat the Great Demon Lord" the actual focus is on the princesses and their interactions with each other, meeting other princesses for the first time, forging alliances, making friendships, in fact, and this was kind of disappointing for me at first, then it became a non-issue,
Finally, there's the "punishment" mini-game. I thought this was going to be like Senran Kagura Reflexions but it ended up being a bit more simple, this will either be the main draw, or what makes you avoid this game like the plague, personally, I had fun with it but YMMV, however, do know you can skip it if you'd rather just play the rest of the game without worrying about this stuff. Your final score might decrease a bit, but this can be offset by going for as many elite enemies as possible.
I suspect it was that part of the game along with the... suggestive voices that accompany it, that resulted in Duel Princess being removed from the eShop, AFAIK, Steam also rejected the game, you can now get it via Fakku or DLSite.
Overall, I'd recommend it, fun, cute and wholesome story, a wide variety of characters, simple but entertaining gameplay, and a nice bonus with the punishment mini-game, if you're into games like SK Reflexions, this might be up your alley.