Official Decree is a written note found in Dishonored.
By Order of the Lord Regent
Effective immediately, all access to Dunwall Tower is hereby restricted to official traffic, via water lock only. The front gate is sealed, with no exception.
This order issued from the Office of General Tobias with the full authority of the Lord Regent.
It can be found in the barracks to the left of the tower entrance and below the tiered wooden pathways in Return to the Tower.