To make tower placement more intuitive and precise, I would suggest some placement style known from almost any RTS. Here a little example from Broodwar.
In words:
when "building tower" is selected and the mouse is hovered over the map, you see where the tower would be placed at the current mouse position. You see the transparent image of the tower snapped to the grid position that fits most (derived from current mouse position).
Implementation style:
show tower image snapped to the grid + show radius of the tower (thin white line?)
Mark tower sprite green if it can be build at current position and red if it can't be build there ... OR ...
Show sprite only if placement is possible, and don't show if its not possible
I prefer the first marking style (with green and red), because it shows that tower building is active now even if you are hovering over un-buildable terrain.
To make tower placement more intuitive and precise, I would suggest some placement style known from almost any RTS. Here a little example from Broodwar.
In words: when "building tower" is selected and the mouse is hovered over the map, you see where the tower would be placed at the current mouse position. You see the transparent image of the tower snapped to the grid position that fits most (derived from current mouse position). Implementation style: