Gunther Prozen, the Evil Chancellor of the Guylos Empire and the Big Bad of season 1, keeps a pointless war with the Helic Republic going so that he can seize control of the Empire. Attempting to murder the child emperor Rudolph, Prozen sends his personal killer, a boy he indoctrinated from childhood named Raven, after him while also sending "rescue squads" that he has Raven destroy in order to cover himself while he continues the war. Prozen then attempts to awaken the ancient weapon the Death Saurer and demonstrates its power by attacking his own capital. Even after defeat, Prozen adopts the identity of the Dark Kaiser with intention of re-awakening the Death Saurer and wiping out organic life.
The second Big Bad of the series, from season 2—Guardian Force—Hiltz is an Ancient Zoidian who desires to exterminate all human life and rule Planet Zi himself. Initiating a series of terrorist attacks to destabilize the hard-won peace, Hiltz kills any men who fail him and even tries to kill his right-hand woman, another Ancient Zoidian named Reese, just because she's no longer useful to him. Revealing his secret weapon the Death Stinger, Hiltz proceeds to bombard civilian targets and announces that when he proceeds to the Guylos Empire, he'll destroy every civilian town and city he sees along the way. Finally fusing with the Death Saurer, the sadistic Hiltz dedicates himself to exterminating all organic life on Zi, gloating how he will be the one to rule over everything.
Crowning Music of Awesome: The music for the anime is some of the best around, and it's used frequently for non-fiction history and military shows in the US.
Draco in Leather Pants: Raven is an egomaniacal bloodthirsty killer who starts off as Prozen's attack dog, and later goes rogue and becomes a terrorist. He abuses and belittles Shadow, wants to commit genocide against all Zoids, and will even murder his own allies; yet we're meant to feel sorry for this jerk because he has a tragic backstory. Predictably, he has tons of fangirls.
His Heel–Face Turn near the end of the series, his sincere grief and regret over his actions when Shadow is killed, and his Pet the Dog moments with Reese probably have a lot to do with this as well.
Ensemble Darkhorse: O'Connell has quite the fanbase in the US despite having a relatively minor role.
Fan Nickname: "Guardian Force" for the Post-Time Skip episodes. These episodes were still officially titled "Chaotic Century".
Hilarious in Hindsight: In one instance Fiona is caught by one of Rosso and Fiola's henchmen, who comments that she's cute and will be a "real knocker in few years." Come the timeskip in Guardian Force, we see he was right on the money.
Idiot Plot: Chaotic Century suffers from a SERIOUS case, to the point where the plot moves forward not because of the skill or resourcefulness of the protagonists, but because of the stupidity of the villains.
Tear Jerker: When Irvine's Command Wolf bites the dust after being hit by Raven's charged particle cannon. More specifically, the monologue that Irvine delivers to the dying Zoid is what pulls the heartstrings.
It gets better though, in the form of a Mid Season Upgrade.
To this Troper, Rudolph's blood-curdling scream when Van's Shield Liger gets destroyed.