And plus, this is just my idea. On every level we gained we have points to use. Every level give 1 point. This points use for upgrades. The upgrades are;
decresed the price of tower inside the battle 0.2% (it cost 2 point)
boost damage 0.2%, (it cost 2 point)
add more life on the base 0.4% (it cost 2 point)
increased the money by 2% everytime clearing a stage (it cost 2 point)
have an extra slot for troop (it cost like 70 points)
I already estimated this .So in every 10 level you can make 1% boost of your choice (boost damage or decreased price.) And in 100 level you can make a total 10% boost. While the more life upgrade are 0.4% boost every upgrade because some enemy are really high life. So Every 100 level you can make a 20% addition on life of the base. And Its just paid the hardwork. Because its really hard to levelup. And we really need some boost when we just play solo and also from doing a very hard event.
And also, the increased money are 2% every upgrade. Because we really need a lot of money. So even we didnt need to use farm now. And its not that much. Because on every 100 level its just give a total of 50% boost on money.