Hard to define this exploit, aside the niptick on the 15m cooldown, but what if the other GM actually waited 15m ? Learn from it, CTG is one of the strongest skills a guild may have. What about your fort in-defence ? When an entire guild gets in, it's flamethrowers' best time, unless too confident in the positioning atop the mountains you forgot to put them.. (and that would be even a worst error than not foreseeing a CTG in your base). Next time will be better.