Doghbarum was a metropolis in Palgrost that neighbored the capital, Balgadur. It was also in this main city that Boron had set his camp while waiting for the Liguritudum army to arrive.
He wasn't worried if his brother, Thelgrun sent full force at him. In a direct clash, he won't win against his brother's force that had more than twice his army. However, if he played defense within the city wall, Thelgrun wouldn't be able to defeat him in a short time.
If his brother brought siege weapons, all he had to do was retreat to a further town. Although Doghbarum was a neighboring region to Balgadur, it still required around two days of travel for the army. If siege weapons were brought, it would require more days.
Boron just needed to wait for the Liguritudum army to get close enough before he coordinated with them and advanced together to Balgadur.