The emperor ordered everyone to prepare for the incoming beast tide. The odds might be against them, but they should be prepared when the savage beasts will come.
All the nobles became very busy as they designated orders to their servants and subordinates. Although they felt reluctant to send their soldiers, they didn't have a choice. If the Celestial Sword Empire will fall, then all the wealth and authority they gained will be for naught. Thus, to preserve their identities as nobles, they sent their soldiers to prepare for the incoming beast tide.
The Death Forest was located in the northern part of Sword City. The purpose of building their capital city in that place was also to fend off the beast tide. Only the capital city had the power to repel hundreds of thousands of savage beasts. Thus, the first emperor decided to build Sword City in that location.