In one variation, White delays pushing their rook pawn (whereas Black delays the rook pawn push in the modern era) and there is an early bishop trade before both players' kings are moved into their castles.
In the 1947 Meijin tournament, Masao Tsukada and Yoshio Kimura played an early example of the typical 3.
Blocking White's bishop with a silver (and also defending the eighth file from a possible attack) is an older Fortress joseki, which typically also includes advancing Black's rook pawn.
At move 6, White also has an option to play Climbing Silver by advancing their rook pawn to rank 5.
Additionally, earlier Fortress openings in the first part of the 20th century did not follow from the standard 1.
At this point, both players may convert their left side configurations into Fortress castles by pulling their bishops back and moving their silvers to the 77 and 33 squares.