Middle Ages

The, Chūsei (中世) are the year 600 A.D. in. It is the first time period visited by Crono and his friends.

Geography
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, Magus' Castle) 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.

Gates
Only one gate leads to 600 A.D., located in Truce Canyon. It connects to Leene Square in 1000 A.D.

Another massive gate briefly emerges when the team defeats Magus, and Lavos is prematurely awakened. It grows magnitudes larger than a regular gate and swallows Magus' Keep and everyone inside, who at this point consist of only Crono, his two friends, and Magus himself. It drops Crono and the crew in Prehistory, while Magus is transported back to Antiquity. The gate is very unstable and closes immediately thereafter.

Native Characters
Frog is the only playable character native to 600 A.D.

Other notable characters from this time period are:
 * Queen Leene - The Queen of Guardia.
 * King Guardia XXI - The King of Guardia.
 * Chancellor - King Guardia's right-hand man.
 * Commander - Captain of the Guardian knights, and brother of the Master of Kitchens.
 * Master of Kitchens - The Chef of Guardia Castle, who provides rations to the troops at Zenan Bridge.
 * Toma - An explorer who offers Crono information in exchange for payment for a drink.
 * Tata - The Legendary "Hero"
 * Masa & Mune - The living spirits of the magical sword, Masamune.
 * Magus - Leader of the Mystic army.
 * Ozzie - Former leader of the Mystic army, takes over after Magus disappears.
 * Flea - Enchanter of the Mystic army.
 * Slash - Master swordsman of the Mystic army.
 * Fiona - Young woman bringing green back to the southern desert.
 * Marco - Fiona's husband, recently returned from the war.
 * Cyrus - A hero slain by Magus in 590 A.D. and buried by Frog in the Hero's Grave.
 * Yakra - A fiend that takes over Manolia Cathedral.
 * Carpenter - A man living in Choras who will repair the Hero's Grave, but he has lost his tools.

Story
Crono and his friends make several major visits to this time period during their quest.

1st Visit
The first visit comes as a result of the accident involving Lucca's telepod at the Millennial Fair in 1000 AD. As a result of the accident, Marle is transported to 600 AD. Crono follows her shortly after, followed by Lucca. Upon reaching 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 3 rescue 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 3 friends travel back to the present the way they came.

2nd Visit
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 this 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.

3rd Visit
The third visit to 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 Magus' 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 gate materializes and pulls the heroes into the Prehistoric era.

Side Quests
There are multiple side quests which 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.

Ozzie's Fort
Located in the center of the map, Ozzie's Fort is a hideout for Ozzie, Slash, and Flea following their defeat by the heroes in Magus' castle.

Sunken Desert
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.

Giant's Claw
The Giant's Claw is a large cave on an island in the south central area of the map. By talking to Toma 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.

Sun Stone
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 Spiced Jerky to a lady in Porre for free. As a result, the future resident will be more generous and return the Sun Stone.

Lost Sanctum
600 A.D. is one of two locations of the Lost Sanctum. It is located northwest of Ozzie's Fort.