Complex sentence: The bireme was twice the triaconter's length and height, and thus employed 120 rowers, while the trireme, keeping the length of the bireme, added a tier to the height, the rowers being thus increased to 180.
Split into simpler sentences: The bireme was twice the triaconter's length and height, and thus employed 120 rowers. The trireme, keeping the length of the bireme, added a tier to the height, the rowers being thus increased to 180.