Look at Prime World: Defenders!

Prime World Defenders is a tower defense game with some RPG elements such as leveling up towers and collecting items to strengthen them, getting experience to unlock talents and grinding to get more gold and levels to be able to beat the most difficult ones. It required too much grinding for me.

Here's what the game looks like!

The basic gameplay in PWD is solid. You start a level with some amount of energy and spend it to build towers. You have a bar with a few tower choices that you can change between battles and some spells that don't cost anything but have a long cooldown. Then, enemies come marching through the level (either a straight line to an exit, a back-and-forth path between resources and the exit, like in Defense Grid: Awakening, or through a maze that you need to build up), some enemies with invisibility, some enemies fly, some heal their allies. If too much enemies go through, you lose. There are special spots where your towers get increased stats (damage, range, attack speed) and evil totems that heal or increase the speed of your enemies.

Plenty of reasons to restart story maps.

By completing levels, you get experience and coins. You also get stars for preventing enemies to get through the mazes (3 stars if you got it perfectly, 2 stars for half your lives lost, 1 star if you barely make it). You also can complete map-specific achievements such as finishing a level with a lot of flying enemies with no more than one anti-flying tower, or not casting magic. These goals often don't mix well with getting 3 stars, so you'll want to redo the maps multiple times. Coins are used to buy things in the store and experience gives you levels.   You get new talents at level 1, 3, 5, 8, 12, 16 and 20 but besides that, what good is the experience for? Leveling from 8 to 9 feels pointless when you're not getting better in anything.

Some of these are useful, some are less so.

Coins are stars are much more important, you need to buy card booster packs to strengthen and fuse them with your existing tower cards. You can either strengthen your towers to increase passively their damage/speed/effects by fusing them with other similar towers and artifacts, special cards that are only used to level up your towers or fuse them with identical tower to upgrade them. by doing so, during battles you can pay to increase their attack for that one battle. The game says it's more efficient to level them up than to place more towers, and I mostly agree.

You can't fuse anything with anything, but most towers and spells are mixable with other towers and spells.

The game feels too difficulty, I feel like you should at least be able to 'beat' the levels without getting 3 stars and the achievements without grinding too much but the boss stages and some regular ones are making me fail repeatedly. Bosses will usually kill you in one hit if they cross your maze and their huge HP pool combined with the stream of enemies coming in makes it obvious that you need to fuse your tower, buy better ones, get new spells and get more talents to win as the game goes on. The game tries to accommodate the grinding by offering 3 side-levels for every major story mission, these side levels are repeatable infinitely and you have a choice between easy, normal and hard. You get gold and experience and a random card from a choice of 5, but this gets repetitive fast.

A few choices. The levels are randomly generated and extremely similar.

I wish the game had difficulty settings before each levels, that you could win 3 stars on the hardest difficulty, 2 on the medium and 1 on the easier one. I'm just stuck at a boss that regenerates health constantly and I don't want to grind some maps until I can level my towers enough. This is too bad because there are a few good ideas in Prime World Defenders, I like tiered achievement system that give you rewards when you complete them, I like collecting things and fusing them, I like talent systems, and yet presented with the option of running the same levels a few too many times until my dragon towers get +10% damage and NOW I can beat the boss, I just want to stop.

If this was an iPad free to play game, stars would be the premium resource.