I had some fully-upgraded towers built near each of my map's four spawn points. After I sold them, enemies near the spawn points continued to take damage.
Someone might want to test whether selling towers does anything other than give you money and allow you to build a new tower in its spot. :)
I sold each one of my towers at level 200 and continued to beat each wave, towerless. To compensate, it seems that the amount of money you get from selling a tower is nowhere near the stated amount :).
Someone might want to test whether selling towers does anything other than give you money and allow you to build a new tower in its spot. :)