Herod introduced numerous architectural innovations and construction techniques in his buildings, such as the domes inside the Double Gate to the Temple Mount. He adapted the mikveh—a Jewish ritual bath—for use as the frigidarium in the Roman-style bathhouses in his many palaces. Herod also developed an innovative combination of palace and fortress; examples include the Antonia Fortress in Jerusalem, the Herodium in the Judean Desert about 2 miles south of Bethlehem, and Masada. Characteristically, they have (or had) one tower higher and stronger than the others. Herod's fortification innovations strongly influenced the military architecture of subsequent generations.