Kingdom of Guardia
The Kingdom of Guardia was founded in the year 1 A.D., and would last nearly a thousand years. It was possibly founded or built with the aid of the Frozen Flame, according to legends of a northern kingdom holding the artifact. The Kingdom was led by a monarchy and was formally protected by the Knights of the Square Table. At various points in history, Porre, Choras, and Dorino have claimed to be protectorates of the Kingdom of Guardia. In the Mystic War, all were united against Magus. However, these claims of loyalty may have eroded as the modern age took root. In the year 600 A.D., Guardia became embroiled in its first known war, the Mystic War against Magus. The warlock destroyed many of Guardia's troops, and nearly defeated the powerful kingdom. However, he was defeated in one of two ways depending on the timeline. In the Lavos Timeline, Magus summoned Lavos to exact revenge but was presumably killed. The Kingdom previously foiled another plot by Yakra to kidnap Queen Leene. Without their leader, the Mystic Army fell into disorder and ruin and were defeated by Guardia. In the later Keystone Timelines, Crono and his party battled their way into Magus's Lair, intending to use Frog's Masamune to strike down the sorcerer. They succeeded, but Lavos activated a massive gate which sent Crono's party to 65000000 B.C. and Magus back to his home kingdom of Zeal. Without their leader, the Mystics were defeated.
Guardia then continued peaceably until 1000 A.D., during which the strained relationship between Princess Marle and King Guardia XXXIII began to boil. After Marle snuck out of hte castle and disappeared to the past with Crono, the Chancellor (Yakra XIII in disguise) scheduled the execution of the boy. Fortunately, Crono escaped with the others into 2300 A.D. Marle later returned to make amends with her father, and foil Yakra XIII's efforts to have the king deposed. Crono and the others were subsequently welcomed in the castle; Crono later married Marle, becoming next in line for the throne.
There is no evidence of any complex machinery or industrialization in the Kingdom, yet Lucca has the capability to design and build a machine capable of artificially bending spacetime to create portals to other eras, something that has been pinned by modern astrophysics as either washed-up science fiction or, at best, strictly theoretical at this point. Not a single gun is seen in Guardia in the original game, either, leading one to believe that they have not developed the technology yet -- which makes a certain amount of sense, as Guardia is a peaceful kingdom. This particular lack of technology may be the reason for their fall; early guns are consistently seen in the Porre military's arsenal. For the most part, Guardia's arsenal, like its culture, appears to be fairly medieval in style, drawing on classical tales such as the Arthurian legend.
Guardian culture appears similar to that of the high Middle Ages with a modern Western twist. This is witnessed in the Guardian courtroom when Crono is tried for the kidnapping of Marle. The judicial system is almost exactly the same, but there does not appear to be any kind of constitutional document or parliamentary body, unless Crono is actually being tried before the Guardian Senate rather than in a public trial as it appears to be. Guardia's religion is also an open question; there are scattered references to both Judeo-Christian and Eastern religious traditions. The sole piece of evidence for any variation of the former being Guardia's religion is the pair of crosses seen on the Cathedral in the PSX version of the game; the dating system appears to be situated around the founding of Guardia rather than the birth of Christ. The latter is more in line with the metaphysical tendencies of other Square games, most notably several Final Fantasy entries, and in Chrono Cross it is confirmed that the planet possesses sentience. The flag of Guardia seems to depict a dragon or Dactyl.
Regents and Cities
In 1005 A.D., Guardia fell to the Porre military as depicted by a cut scene and spoken of by Radius of the Acacia Dragoons. Dalton was behind the fall as revealed in Chrono Trigger DS; his plan was to exact revenge on the team. While Crono and Marle were ssaid to have fallen victim to some kind of incident, they almost definitely survived as evidenced by Lucca's later remarks to Kid. In 1015 A.D., Lucca was either killed by Lynx or killed herself in Another World in order to keep the Prometheus circuit lock on the Frozen Flame intact. The fates of the Chrono Trigger Team are directly addressed at this article.
Three things suggest that Guardia was able to rebound from this fall in 1005 A.D. Firstly, Radius speaks of Guardia as if it were still a sovereign nation. Secondly, Norris reveals when he approaches the Frozen Flame that Porre requires it to surpass the other strong countries of the world. Lastly, in the ending General Kid, Kid declares that the first order of business for the Dragoons is victory over Guardia, and then Porre. This suggests that Guardia did make a comeback, and that perhaps Crono and Marle are still alive.
Guardia is possibly alluded to as a northern kingdom that kept the Frozen Flame for many years.