Gwanggaeto's conquests are said to mark the zenith of Korean history, building and consolidating a great empire in Northeast Asia and uniting the Three Kingdoms of Korea under his influence. Gwanggaeto conquered 64 walled cities and 1,400 villages. Except for the period of 200 years beginning with Jangsu, who would build upon his father's domain, and the golden age of Balhae, Korea never before or since ruled such a vast territory. There is archaeological evidence that Goguryeo's maximum extent lay even further west in present-day Mongolia, based on discoveries of Goguryeo fortress ruins in Mongolia. Gwanggaeto established his own era name, "Yeongnak Eternal Rejoicing", proclaiming Goguryeo monarchs equal to their counterparts in the Chinese mainland.