For A First Game This Is Surprisingly Good.
To start off with, the XP/Levelling system was a nice touch as its something you don't see very often with conventional Tower Defence games, I also feel the difficulty for the first 2 maps was very well balanced and gives a good insight to the player on how they will have to approach each round. The game itself runs smoothly with no issues or stuttering/lags as you would usually get from games of this budget.
But I've been mainly asked to give criticism so here are some nit-picks.
1. On Map 1, There are a few tiles at the top of the map that were inaccessible (IE: you couldn't place any towers) perhaps the developer assumed that the player wouldn't use every tile in the first map and simply decided not to make them accessible, I then assumed it was maybe just a design aesthetic but them realising that all the inaccessible tiles were all together in the same location I came to the conclusion that it wasn't the reason.
2. Upgrades, The cheapest tower available for you to purchase is 250 Units and the upgrade for that tower is 400 Units, The most expensive tower is 500 Units but the upgrade is... 500 Units, This is just a 100 Unit increase compared to the upgrade to the cheapest tower in the game, Now if the upgrade cost for the first tower is approximately 46.1538461538% then the tower that costs 500 Units should have the upgrade valued at 730 Units (Or 750 For Simplicity), In Conclusion, I believe that the upgrade cost for Tower 1 Should be either lowered, or the upgrade cost for the most expensive tower be increased in line with that 46.1538461538% calculation.
Buuuuuuuut thats just a theory, A GAMMMMMMEEEEE THEORYYYYYY
Thanks For Reading :D