Complex sentence: In the Middle Ages (recorded from around 1200 C.E.) a less powerful version of the onager was used that employed a fixed bowl rather than a sling, so that many small projectiles could be thrown, as opposed to a single large one.
Split into simpler sentences: In the Middle Ages (recorded from around 1200) a less powerful version of the onager was used that held the projectile in a fixed bowl instead of a sling. This was so that many small projectiles could be thrown, as opposed to one large one.