As far as municipal iron castings are concerned one of the most well-known product would be the cast iron manhole covers. Everyone knows these are the ninja turtles front door but they are also a cleverly designed entry point that allows for passage, supports traffic, and will never fall into its hole. The circular design exists because it prevents the cover from falling into the hole, pretty cleaver if you ask me. Cities and towns use a variety of sizes and often have standards of denoting what the manhole cover is being used for.