Fortified Wall is a type of defensive structure available on Age of Empires II once you reach the Castle Age. It is obtained by researching it at the University. It offers considerable defense to a player and is nearly impenetrable without siege weapons.
The fortified wall was an upgrade of the stone wall accomplished by building fighting positions into the wall so that defenders could be more effective in repulsing assaults. Strong points and bastions allowed defenders to shoot at enemies at the base of wall, where they might otherwise be safe. The three lines of stone walls outside Constantinople were fortified with hundreds of towers, helping to make the defenses impregnable until the advent of cannon.