The Norbs are coming to attack your orbs and it is up to you to stop them! Build up your defences to push back the Norb advance; if your orbs are lost, you will fail! Construct your turrets to best defend against the invasion and upgrade them to provide better fire support. Watch out, though: Awesomeo, leader of the Norbs, is waiting...you had better be prepared.
Information on each individual tower can be found in the article.