The magi have identified several vis sources on the island. The vis sources below have been officially registered with House Mercere as belonging to Castrum Viribus.
- Donnchad's Tomb: 4 pawns of Rego vis collect annually in the tomb's lid.
- The Carnane: This menhir on the west side of Man has bowl-like depressions carved into its base. After powerful storms, a pawn of Aquam vis can be collected from the water within these bowls; the vis must be harvested within a day, or the water evaporates. An average of 4 pawns a year can be collected in this way.
- The Cave of the Carpenters: 4 pawns of Aquam vis collects annualy in the coral within the cave.
- Orlaith's Ashes: 4 pawns of Ignem vis can be collected from the ashes inside the vase every year.
- The Round Meadow: Fenodyree revealed that herbam vis grows in the grass of the meadow, but the magi have not yet tried to harvest it.