We have those giant guns in Twilight Gap (Destiny)

by Ragashingo , Official DBO Cryptarch, Thursday, May 14, 2015, 00:35 (3718 days ago) @ Chewbaccawakka

Places --> The City --> The City Age:

As the City learned to walk again, it found a world overrun by alien menace. It faced disaster and defeat. Even in recent years, as Guardians begin to venture back to the Moon and the inner planets, the City's territory has withdrawn - outer sections abandoned and converted into fortifications in the wake of the Battle of Twilight Gap.

Various details about the battle are scattered about. How The City's guns exchanged fire with Fallen Walkers, how Titans, Hunters, and Warlocks all did their best to stop the Fallen advance and failed, how it was the House of Kings (I believe) that rallied all the houses together, etc. It's all in there unless I'm badly misremembering something. :)