Chain Barrier

The Chain Breaker rose higher, eventually cresting the wall of the floating stronghold and coming to a stop above the ramparts. From there, they could see a bit further into the haze. 

Far away to their left, another floating fortress could barely be seen in the dim twilight, just as battered and forlorn as this one. There was one more to their right. All three had been connected by thick chains once, but those had long been torn.

Sunny remained motionless for a moment. 

'...It's a barrier to block the Great River.'

There must have been a ring of these fortresses built around Twilight once, with massive chains strung between them both above and beneath the water. That way, the Defiled would have had to break through the chain barrier before reaching the city.