Upgrade Mark (BTD4)

The Upgrade Mark is a mark in BTD4 that signifies the tower has been upgraded. Some towers have no upgrade marks when you upgrade it, like Banana Farm. Some towers like Ice Tower have all level upgrade marks except for the final level. And some have only 2 upgrade marks like Dartling Gun, Dart Monkey and Tack Shooter. The upgrade marks disappear when the tower's appearance changes. 

Trivia

 * In BTD5, upgrade marks never appear, replaced by minute changes in the appearance of an upgraded tower.