OK. You could have tiles that when touched or clicked would change to the tower or you could drag a tower and it could only be placed on a placeholder object. Or in a grid position. It depends on how you want the player to add the object. Either touch a location or drag from a tray or a combo of both.
You can search the forum for grid or drag grid.
I didn’t download the project bc I’m on my phone but this has an interesting drag demo.