Millenarian is a powerful and intense dose of technical metal. Fuzzy riffs and complex arrangements meld into more traditional metal shredding. There are long passages of instrumental sections punctuated by muffled yelling style vocals. Many comparisons have been made between American Heritage and Mastodon, and their styles are definitely similar, although American Heritage is not a clone and have their own distinctive approach.
American Heritage has tons of quick, angular riffs, but is backed up with some heavy thrash and a lot of loose bottom end crunch. American Heritage sounds as heavy as hell by just playing super tight, super intricate metal - but don't let their technical prowess lead you to believe these guys are "tech." The vocals take a background and the music itself just rides and carries the weight. But I don't mean this is like Pelican with some vocals, it's an integral part, but not the focus. I liken it to a band like Akimbo or Lords hanging out with the dudes and dudette from Kylesa or His Hero is Gone rocking out to Anodyne and playing some raging metal. And, to me, that's what it boils down to.