The fortress was constructed by King Korsikk of Westmarch in order to pen the "Barbarian threat" to the north. After years of military disasters, the frustrated king led a large army out of the fortress and into the north to conquer the Barbarians once and for all. None of them returned. Since that time, Bastion's Keep has remained as a bulwark between the violent tribes of the north and the lands of the south. The soldiers who manned its walls remained ready to confront any foe, human or otherwise, who might besiege them.
The walls of the keep were damaged from the explosion that stemmed from the destruction of the Worldstone. Even two decades later, the damage had yet to be repaired.
The Siege of Bastion's Keep
"You thought you were so clever, that you had outwitted us all. One by one, our brethren fell into your trap, but not me. I defy you! I now know the Black Soulstone is the key, and it shall be mine. Soon, my armies shall pour forth from the Shattered Mountain, ravaging this world and all hope of resistance. My minions will find the stone, wherever you choose to hide it. Then, at long last, Azmodan shall reign as Prime Evil!" - Azmodan's declaration
In 1285, the fortress's impregnability was put to the test as Azmodan and his Army of Sin moved south from the Arreat Crater, the crater itself serving as staging grounds. The demons' assault was relentless, they seemed without number, and kept up their attack through both night and day. Morale among the besieged defenders dwindled. At some point, a request for reinforcements was made of Westmarch.
On the ninth day of the siege, Commander Calderos decided that a "righteous show of force" was what was needed to send Azmodan's army into retreat, and intended to lead a charge from the fortifications the following day. It was a tactic that did not work and the assault continued.
Defeat neared. Bastion's Keep was close to falling, and Azmodan's minions knew it, as many demons paused in their assault to feast upon the flesh and blood of their mortal foes. Azmodan called an end to their indulgence and ordered a final assault, wanting the keep to fall before the next dawn. By the time Tyrael and his allies arrived (having previously imprisoned Belial in the Black Soulstone), its soldiers were scattered, and Azmodan was throwing everything he had against the keep. Legions of demons crashed against the walls, sending tremors through the bones of the fortress, and fiery projectiles rained down against the walls. That the damage from twenty years ago had yet to be repaired didn't help matters. Tyrael, taking command of the keep's defenders, ordered the Nephalem to light the keep's signal fires, which served to boost the defenders' morale Azmodan was aware of the Nephalem's presence, but kept up the attack nonetheless.
The Nephalem made their way across the keep's battlements successfully lighting the signal fires even as demons spilled over the walls. The Nephalem met with Sergeant Dalen, who informed the hero(es) that the catapults could break Azmodan's lines, but couldn't be raised due to the presence of demons on the walls. Afterwards, the Nephalem, with the aid of the keep's soldiers, was able to raise the fortress's catapults into position. With that done, projectiles rained down upon Azmodan's army.
Azmodan mocked the Nephalem, revealing that while the catapults were being raised, his forces had breached the keep in its lower levels. To make matters worse, a messenger arrived from Westmarch, informing Tyrael that King Justinian would not send reinforcements to fight "imaginary demon lords." Tyrael bitterly commented that the king might believe when his kingdom was burnt to ashes around him.