It was previously established that the fire from dragons could destroy stone buildings. Remember Harrenhal?
This was all that was left after Aegon Targaryen and his two sisters attacked Harrenhal during the War of Conquest
Harrenhal wasn't attacked by death lasers. Aegon charged at them from above with his dragon and everything that was flammable within the castle started to burn. The towers started to glow and even partially melted from the heat during the span of a night. Then no one after held it long enough to maintain the castle, which didn't help it too.
Even GRRM, the man who apparently thinks stew pots can melt gold, was sensible enough not to give dragonbreath the thrust of a rocket engine. The CGI was done for a pretty, but dumb spectacle, worthy of Michael Bay, and that was it.