I was under the impression that the tradition is that he build prefabricated bits and pieces and assembled the fortress by just joining his pre-built parts. Certainly Sunomata Castle as it stands now is not the one that Toyotomi built--his "castle" must have been little more than a stronghold, a structure composed mainly of rudimentary wooden palisades and wooden towers. A fort appearing in such a short time right across the river in a strategic position would surely surprise an entrenched enemy that relied entirely on his ability to prevent the enemy's maneuver by using his fortified position