As the title says. You can build towers; however, once you add a second tower, the first tower can no longer be upgraded (upgrade inventory is blank). If you build and upgrade the same built tower, then it'll be okay... but if you build another tower (with the original T2 tower), then the T2 tower will only show its own upgrade. That bugged T2 tower will no longer show the other potential towers it can upgrade to. Building the single attack neutral or bomb cluster tower affects the element towers as well.