The Coldest War
The Locks were once engineering marvels that permitted ships to travel between the Sound and the higher elevation of Lake Union. During the Madness, two operators of the locks managed to maintain their sanity and keep the machines working to accommodate the flood of boat-owners seeking to flee the burning city. After the initial deluge, though, the Locks were abandoned as the survivors had more urgent concerns.
About a decade later, however, when the Esoterics began crawling out of the sea, a tribe of people, inspired by Pike’s Watch, gathered around the old Locks, killing what malformed creatures had already entrenched themselves there and establishing defensive fortifications. These Engineers may not actually know much about engineering, but their efforts have kept Lake Union largely free of Esoteric incursion. Should the Locks fall, nothing would prevent the twisted horrors from the sea from crawling down into Seattle from the north.