Holy Sanctum is a small dungeon that can be found in Gardens of Hope 2nd and 3rd Tier in Act IV of Diablo III.
Inside, an event named Watch Your Step is guaranteed to occur, if the zone itself appears.
Town Portal cannot be used within the Holy Sanctum, possibly because the unstable ground of the Watch Your Step event would cause potential problems. There is no restriction on leaving the Sanctum, using Town Portal, then re-entering it.