The city itself is a maze of mechincal halls and tunnels, and consists of many levels ranging from living quarters to shopping malls to factories. One of the lowest levels is the geonyte power plant, which absorbs geonyte directly from the earth and lets it flow underneath a glass floor. Not far above that is the arms manufacturing plant that you visit in Monster Rancher EVO. It's safe to say that one of the levels also serves as their arena or coliseum.
Grabad is one of the Orcoro Circus stops in Monster Rancher EVO, during which time they are experiencing a geonyte shortage, and it's also a place that can be explored in My Monster Rancher. Sometime during that time lapse, the lowest levels of the city that house the factory become overrun by wild monsters and the geonyte production is shut down.