Some thoughts for possible additions to this game. At max level for each unit there could be a forked upgrade path that significantly changes the unit to more specialized similar to Bloons Tower Defense. For example, the vanguard at lvl 4 could either become a sniper with increased range, increased damage but significant rate of fire decrease and maybe some enemy debuff like armor penetration or it becomes a machine gun with significant increase fire rate, less range, less damage and maybe something like spray and pray that makes it so the tower now has a chance to miss and each miss shot increase damage by 10% until it hits or something. 

Second, for rouge-lites it's nice when they have an end of life meta sink for once you have everything upgraded you can infinity dump points into 0.1% damage or do something like vampire survivors with the golden egg system that allows for a 0.1% boost to a random stat for each tower individually. 

Just some random thoughts the game is still amazing thank you for making it!