Very cool concept, and very fun game too, mixing tower defense with incremental / bullet heaven theme but also taking synergy into accounts is a nice idea, with some polish I can see the game getting very addicting, typical of these kinds of games.
One thing I would suggest is making the "drag and drop" mechanic for the towers either more clear (add a tutorial, even a simple tutorial text that says to drag and drop the icon on the arena) or maybe replace this mechanic with a click first (to buy) and then click on the arena to place them. It can be confusing seeing a shop with tiles and a price underneath, which prompts the player to try to click tiles to buy them instead of holding and dragging the icon to actually place it.
Also, to fit the theme of "expanding" a bit more, maybe a way of expanding the area in which you can place tiles would be ideal, for example leveling up your central tower to place tiles in a 4x4 or 5x5 area instead of a 3x3 area.
Very fun concept overall, very nice game :)