The geography of 600 A.D. is most similar to that of Present. These 2 time periods share a majority of major landmarks with only a few exceptions. 600 A.D. has the most places of interest of any time period in the entire game as it is central to many major plot events (Manolia Cathedral, Denadoro Mountains, Fiendlord's Keep) as well as several side quests (Northern Ruins, Sunken Desert, Giant's Claw). It also possesses the most locations where enemies can be fought in the entire game.
Another massive time gate briefly emerges when the team defeats Magus, and Lavos is prematurely awakened. It grows magnitudes larger than a regular time gate and swallows the entire Fiendlord's Keep and everyone inside, who at this point consists of only Crono, Frog, the third party member, and Magus himself. It drops Crono and the crew in Prehistory, while Magus is transported back to Antiquity era. The time gate is very unstable and closes immediately thereafter.
Frog is the only playable character native to 600 A.D.
Other notable characters from this time period are:
Crono and his friends make several major visits to this time period during their quest.
The first visit comes as a result of the accident involving Lucca's telepod device at the Millennial Fair in the year 1000 AD. As a result of the accident, Marle is transported here. while Crono follows her shortly after, meanwhile is followed by Lucca. Upon reaching the year 600 A.D., Crono finds Marle has been mistaken for her ancestor and identified as Queen Leene. The real Queen, having been kidnapped by Magus' troops, is still being held captive. Marle speaks with Crono just before she vanishes from existence. After Lucca arrives, she explains to Crono that unless the true queen is rescued, that Marle will cease to exist in the future, thus explaining her disappearance. Crono and Lucca track the queen to Manolia Cathedral where they are joined by Frog. Together, the three rescues the queen from Yakra who was posing as the royal chancellor. After rescuing Queen Leene, Marle reappears where she vanished in the castle. Frog leaves the party shortly after the queen's rescue. Using Lucca's newly constructed Gate Key, the three friends travel back to the present the way they came.
The second visit to 600 A.D. comes after Crono and friends learn that Lavos has a direct relationship with Magus. They believe that Magus created Lavos and that defeating him will change the future downfall of the world by Lavos. The heroes find that Zenan Bridge has been fixed since their last visit and aid the knights of Guardia in defeating Magus' troops there. Afterward, the heroes scale the Denadoro Mountains in search of the legendary Masamune which supposedly has the power to defeat Magus. However, upon reaching the summit, they are only able to retrieve a broken portion of the sword's blade. Upon visiting the Haunted Woods, the heroes find Frog's home where he is staying in seclusion. Upon further inspection, they find the hilt of the Masamune in one of these drawers. The hilt is inscribed with the word 'Melchior' a reference to its forger of the same name. The heroes then leave 600 A.D. to visit Melchior and determine how the Masamune can be repaired.
The third visit to the year 600 A.D. comes after the heroes have received the repaired Masamune. They return to visit Frog in the Haunted Woods, convincing him to join them in their quest to defeat Magus. Frog joins the group and they proceed to Fiendlord Castle to confront Magus. However, upon defeating Magus, the group learns that Magus did not create Lavos, but only summoned him from deep in the ground. Soon after, a large time gate materializes and pulls the heroes into the Prehistoric era.
There are multiple side quests that involve 600 A.D.
The Hero's Grave
This Sidequest involves the Northern Ruins location on the eastern side of the map. By retrieving the Carpenter's Tools from a carpenter in 1000 AD, the heroes can commission a carpenter in 600 AD to fix the many pitfalls in the ruins. Doing so reveals multiple treasure chests and allows access to Cyrus' Grave, which can power up the Masamune.
The Sunken Desert is created in 12,000 B.C. by telling a woman in the Zeal Palace or Last Village to plant a tree rather than burn it. Doing so causes the Sunken Desert to appear in the southern desert.
The Giant's Claw is a large cave on an island in the south-central area of the map. By talking to Toma Levine in various towns, the heroes can learn of its location and its Rainbow Shell treasure. Upon entering the cave, the heroes find that it contains the preserved Tyranno Lair from the Prehistoric era.
The heroes must visit 600 A.D. while attempting to charge the Sun Stone as it has been stolen by a resident of 1000 AD In order to retrieve it again, the heroes must give some Jerky to a lady in Porre for free. As a result, the future resident will be more generous and return the Sun Stone.
600 A.D. is one of two locations of the Lost Sanctum. It is located northwest of Ozzie's Fort.