Corpse Slayers Campaign
A massive fort erected centuries ago in the mountain pass that leads to the Empire of Tekilon in response to the invasion that empire mounted. While it is referred to as one fort, it is more accurately described as two forts connected by a wall that is itself a fort, and by underground tunnel in case the wall is breached. Each fort was created out of the mountainside itself and as such, they are very difficult to bombard successfully. The wall that connects them is almost 300 feet thick as well. A single tunnel protected by sniper holes and places for boiling oil stretches through this monstrosity of a wall. The wall itself has places for siege equipment such as ballistas within its stone embrace and is high enough that ladders are not considered a good way to assault the ramparts. In short, the fort took almost a century to build and is considered the most powerful defensive fortification in the known world.