Good concept and content, ok execution
As the game progresses you can upgrade your towers 1 of 2 ways. Evolve them by merging like tower cards together or fuse them which is raising their level. Evolve gives some increase to the tower damage, but it lets you upgrade the tower when placed. Fuse makes your tower stronger but does not give upgrade options during level.
The towers vary is range and ability like bounce attack, aoe, poison, buffs, slows, etc. The game has a lot of different towers with them being ranked as common, rare, and unique. Unique has more interesting abilities, but the biggest difference is you can level common towers to level 10, rare lvl15, and unique lvl25. It takes a lot of cards to upgrade and I recommend only upgrading rares and uniques.
Each level tends to have a different map with different challenges. Before you beat the next map, there are 3 choices for farming, easy, medium, hard. Basically you play on a map you already beat with random monster spawns and at the end you get a card. The point of doing this is a lot of cards is needed to upgrade a single card and you only get 1 or so for beating a map. This is where the problem with the game starts. As to make the game last longer, they have you grind over and over.
As you get more cards and you upgrade your towers, the difference between level 1 and level 25 is too great. For the same cost you have a tower that is 4x stronger. If you evolve it to level 3 and upgrade it during a map, it will be 4x stronger for a total of 16x.
So why is the above soooo bad??? Basically you do not beat the game so much through having the right towers in the right spots or figuring out the correct mazing to the level. You beat the game simply from grinding and getting your towers stronger. If you can beat with less grind and lower tower level, good for you...but it is literally impossible to beat the last guy with level 10 or below towers even if you evolve them all the way. You cant win with maxed out common towers. You need unique or rare towers level 10+. Ideally level 20. Level 25 makes it a cake walk.
Some towers are completely useless. For example stun towers are useless during normal spawns because there maybe 20 monsters in a spawn and stunning 1 creature isnt going to help. It is primarily for bosses, but some bosses like the last one are immune. Slow towers have similar faults for creep waves but do better on bosses. There is an aoe slow but only for ground.
Some towers are op. Most unique are stronger by far than common, which is not a surprise. One thing though is the "prime injector" costs 35 and gives a 25% damage bonus when fully upgraded. Mid to late game it is worth building next to every tower as a single tower will be worth more than the 140 (25% boost times 140 = 35). evolution 1 is 1 square aoe, evolution 2 is 2 squares, and evolution 3 is 3 square aoe. I personally liked the mortar cannon as I was able to mass send waves early. Late game dragon and tesla towers.