You could purchase a tower then change the mouse cursor to be the tower image and then have it appear red where you can't place it and then green where you can place it and then stamp it. Kinda like a rollercoaster tycoon type thing.
If you want to drag the towers from the UI, just give the towers drag&drop behaviour that becomes enabled when they have been purchased. Then you could drag them into the level and set up some logic to snap them to nearest available wall.