Shortly after Belaus Hammertone banished the enemies of the Sanctum, he decreed that never again would the people be taken unawares by the Mire itself.
He staked out a border for the outer limits of Sanctum and had the people construct a wall (The North Wall) as a physical barrier between the safety of Sanctum and the rest of the Drekken Mire. The people are forbidden to cross the Wall to the north for their own safety: while the enemies have been banished from Sanctum, there are still creatures that lurk in the Mire. Only certain members of the the Spears are allowed beyond the wall.
The North Wall is gated and patrolled, but only by one or two guardsmen at any given time. The guards use the vantage of the high wall as a way to watch over their charges in Sanctum itself. The gate is almost always latched, though The Witch of North Drekken never seems to have any problem making her way into the town.