1

 Whole Truth: The Order of the Last Bastion Knights built Fort Frostfell as their first outpost in the valley. They used the stronghold as a place to wage war on the greenskins. The knights abandoned the fortress when they outgrew it and moved to Safeharbor. Their dead are still entombed within.

2

 Partial Truth: The Order of the Last Bastion Knights built Fort Frostfell as their first outpost in the valley.

3

Truth and Lie Mixed: The Order of the Last Bastion Knights built Fort Frostfell as their first outpost in the valley. They left when greenskin shamans put a dread curse over the place that killed many of the folk inside.

4

Lie: The Order of the Last Bastion Knights are secret devil worshippers who built Fort Frostfell as a place to worship fiends in secret. The knights angered the fiends who now control the fort after they ran the knights out. For more information see The Frozen Necromancer: Chapter 4, page 125