What sort of government did the Mound Builders have?
No one knows. There is nothing left of them but the mounds. they were gone before Europeans reached North America.
From various indications, such as the way that they constructed their areas and weapons found, it seems as though they likely did have some sort of military-like defense system. Nothing is known for certain however, as very little is known about the mound builders due to how ancient their society was.