When you build a tower and upgrade it; it does NOT actually upgrade but just take away money.
You can sell a tower two times without it actually disappearing. If you upgrade a "twice sold" Tower; it DOES upgrade. (You can tell if it has been sold twice if it has a red square around it when hovering over it).
For example:
You build a rocket launcher. (cost: 400) You click on the tower and then the sell button. (You gain 300) Nothing happens. You click on the tower and then the sell button again(You gain 300.) When you hover your mouse over it now; You will see it has a red triangle around it.
When it has a red triangle around it; It IS upgradeable and will change colour/looks when upgraded.
When there's NO triangle around it; It is NOT upgradeable and by attempting so you will only lose money; It will say it is level 2/3 but it's appearance stays the same, as does range/attack speed/damage.
The tower will not disappear until the 3rd sell; and by selling it twice you gain more money than the original tower costed.
Rocket Launcher: Costs 400 Selling it twice: you gain 600 +It will not disappear
Sorry to bring back such an old topic, but I've been looking at the bug for a while and gathered further information based on more extensive resources.
When I was making the minigame, I noticed the bug and tried to remove it. I thought it was gone, but it showed up after release. Apparently, when you set a tower, sometimes it makes a duplicate tower on top of it. So, when you sell the tower, you are selling it, but the duplicate remains.
Additionally, it only appears after returning to the menu and playing the game again. It does not necessarily sell twice, but it sells as many times as you go to the menu and return to the game. In other words, there is no limit to how many times you can sell a tower as long as you have repeated this process enough. So, in reality, it's only one bug that appears only sometimes that few people will find, though it is a huge bug.
Using this glitch, I ended up with over 140000 from selling towers before wave 21. Eventually, I'll try to get around to fixing this bug some day, but I'm busy with other things right now. But until then, enjoy the cheat.
