Zuum City is a walled fortress town in the SD Gundam Sengokuden saga. It is a short lived headquarters of Army of Darkness leader Yamishogun. Within its walls resided Yamishogun, Zakuto, the Zaku Band of Three as well as a trio of kunoichi, a trio of ninjas and numerous other guards and personnel including a full force of Zakus. Zuum City is a reference to the original Zum City in the Universal Century and it's fortress shape is deliberately modeled after that of the Zabi Palace.
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.