The Guardians Of Daikaijū City : HADES

When a huge sinkhole opened up in the Hokkaido prefecture of Hakodate, from which giant monsters would periodically emerge, a great walled city, Daikaijū, was built up in a ring around the sinkhole -- nearly everyone who lives in Daikaijū City is either a scientist, an engineer, or a maintenance worker, whose job is to keep the Guardians operating at their peak, so that they're ready for battle at any moments notice with the hellish creatures that rise from the sinkhole!