Later
The enemy army began to move after half an hour. They certainly didn't have time to plan any type of fancy strategy… They wanted to overwhelm Rustburg with superior numbers. In response to that, the cavalry left the town and assumed formation outside the walls. If they were to be attacked by catapults or ballistae, they could react fast with the horses' speed.
Zaos had no idea the effective range of such siege weapons. It changes from the country they come from since their technologies are different. Regardless, even though the cavalry showed itself, they weren't attacked… It looked like the range of the siege weapons was somewhat limited. Unfortunately, Zaos didn't hear about their range in old documents.