When you are underneath one of the towers, click 'X.' Sometimes the dialog box doesn't show up telling us to do that, but clicking on it anyway still works.
If you can't get past this quest after trying to just click X, then contact KI support and report the problem. They will help you get on to the next quest. You can reach them by clicking the "Help?" icon in the upper right corner of this page.
I've found that most times when this happens it's because someone else also has the same quest and has already "burned the towers". You can either wait for the timer to reset or just change realms. I would suggest the realm change as it takes less time.