The Mill is used to grind things into a small powder. You can visit, but the mill itself is locked by a key found in Barbican. You will also need the Bones of Loric.
Bone Dust of Loric[edit]
When you first enter, you will be attacked from the right by archers. As the right-hand route is closed, you have to instead go forward and drop down.
There is only one route otherwise available. Continue down it until you reach two teleporters. The teleporter on the left brings you to the mill. Unlock the mill by the door on the right, and place the bones near the gears. The bone dust will be dropped outside.
Before you leave the mill, there is a mystic urn on the platform facing the mill.
To return, take the other teleporter. To your left (next to the bell tower) is a switch that opens the passage back to the entrance.