Zuum City was built rapidly and boastfully by Yamishogun. His arrogance was even such that he sent a letter of invitation to Shogundam proclaiming his construction of the city and inviting him to send a ninja to verify it. Shogundam sent a ninja there indeed however it was not to verify the existence of the fortress, but rather to uncover the identity of Yamishogun.
The fortress had only been recently constructed when several members of the Gundam Clan attempted to infiltrate it, the first being the brothers Hyakushiki and Hyakkumaru. This duo was able to gain entry into the castle simply by asking the guard, whereas Shogundam's agent Onmitsu had a much more difficult time gaining entry. He had attempted uneccessarily to sneak in and evaded deathtraps in the process.
Onmitsu and the brothers caused a severe ruckas in their battles with the various ninja forces of the castle. In the end a bomb planted by the brothers catapulted the various ninjas from both sides into the tower of Yamishogun. Shortly thereafter Onmitsu was catapulted out of Zuum City as it collapsed.