Rhombus Square is a location in CrossWorlds. It is the capital of Shadoon; a large city of tall glass skyscrapers located on an island off the coast of the Croissant. The main city is only accessible to experienced players who have progressed along the Track of the Ancients to collect the four elements and complete the Trials of the Golden Bugs quest. Until then, it is only possible to visit the tutorial region, Newcomer's Bridge, and the teleportation hub, Cross Central. An additional pass is also needed to access the entire city.
Rhombus Square
Rhombus Square is the biggest city and capital of the human settlements on Shadoon. It is located in the sea near Rookie Harbor and constructed entirely out of glass and advanced metal alloys.
In order for a Seeker to gain access to the city, he must first collect all elements on the Track of the Ancients.
Newcomer's Bridge is a long stretch of buildings with an elevated walkway, located east of the main city. Individual stretches of the bridge are only connected by sections of the Rhombus Dungeon. CrossCentral is a large building containing a teleport hub, from which anywhere in the playground can be accessed. CrossCentral is connected to Newcomer's Bridge via sky rails, a virtual elevated transport system.
Newcomer's Bridge
The Newcomer's Bridge is located in the skyline of Rhombus Square and serves as the basic training for new Seekers. Located near the end of the bridge is the Rhombus Dungeon where the new recruits face their final test before taking the skyrails to Cross Central and leaving for the mainland.
-Encyclopedia entry
Cross Central
Cross Central is the primary teleportation HUB for the human civilization on Shadoon. It is located on a large tower in the middle of Rhombus Square and offers instant teleportation to all human settlements.
New Seekers who just completed the training on Newcomer's Bridge first enter Cross Central to teleport to Rookie Harbor and thus enter the main continent and start the Track of the Ancients.
-Encyclopedia entry
The main city has several large, connected areas, and is not accessible until after obtaining the Meteor Shade. Of particular note are a shopping district in the east and the Arena in the west. The Vestorovich Manor is not immediately unlocked, and is only opened during the good ending storyline. Full access to the rest of the city is obtained at the beginning of Chapter 11, once Sergey gives the Full Rhombus Pass to Lea.
Connecting regions
Rhombus Dungeon via Newcomer's Bridge and Newcomer's Bridge End
Enter from the bottom of South Arch and walk all the way to the right and get to the 2nd floor. Wait for a truck to pass below and jump on top of it, jump on the platform to the north and make you way to the right. Use the High Jump Panel to reach this
One of the main draws of Rhombus Square is the Arena. Here, players can compete in scored matches against previously-fought enemies and win Arena Coins, which can be traded for powerful equipment and cosmetic items. Details on the Arena can be found on its own page.
Enemies
As it is a city, enemies can only be encountered in Rhombus Square in special situations. Only enemies encountered outside of the Arena are shown here.
The Sprout of Ages rare destructible is in the South Arch. From the position where you can destroy the blocks to make the chest of the Very Large Ember accesible, jump north and walk into the southeast corner to reveal the Sprout
Traders
Traders in Rhombus Square offer some of the most powerful equipment in the game, as well as boosters which increase the level of enemies.
In addition to the standard equipment and consumable shops, Rhombus Square has a shop selling backer consumables, a shop selling the Chest Finder, and an arena shop that sells powerful equipment and cosmetics for arena coins.
Rhombus Items (Buy Land GF)
Rhombus Items (Arena Lobby)
Rhombus Items – Identical shops located in Buy Land GF and Arena Lobby.
The A New Home DLC greatly expands upon Rhombus Square, leaving the area almost double the size as in the base game, and with significantly more content.
Layout
The DLC greatly expands upon Rhombus Square with the introduction of the Full Rhombus Pass. While most of the new areas can be accessed immediately, the Tremor Lab is unlocked during the Tremors of the Past quest and the Rhombus Parks Embassy is unlocked during the Wrath of Goat quest.
Enter from the bottom of South Arch and walk all the way to the right and get to the 2nd floor. Wait for a truck to pass below and jump on top of it, jump on the platform to the north and make you way to the right. Use the High Jump Panel to reach this
Take the stairs to the second floor near the road to the East, walk around the house, jump down to two NPCs and cross the road by shooting the Wave orb
After Obtaining the Silver Chest in Cliff Exhibition, continue to the left, walk through the secret entrance to the North Arch, jump down on the platform, make your way to the roof, jump down onto the pillar, continue to the left, jump onto the ceiling, jump down the balcony to the North-West, shoot the Wave orb, destroy both barriers across the road that are blocking the entrance to the chest
From the hole in the fence near North-East corner, proceed to the North-West, shoot the sensor across the road, then get down onto cliffs from the second floor to the East, make your way up, shoot the Wave orb and proceed North
The new quests in Rhombus Square add several new enemies to the area. Only enemies encountered outside of the Arena are shown here, although it is worth noting that the DLC includes one enemy who can only be fought in the Arena, Lily.
New CrossWorlds players pass through the Newcomer's Bridge areas of Rhombus Square while completing the tutorials located in the Rhombus Dungeon, although it is mentioned that this section is skippable for experienced players. After the tutorials, they travel through CrossCentral to reach Rookie Harbor.
Near the end of the Track of the Ancients, Seekers obtain the Meteor Shade, which grants them access to Rhombus Square proper. This is an entirely optional area of end-game content.
A new region of the Track of the Ancients, Azure Archipelago, is implemented later (during the A New Home DLC). Access to this area is only possible through Rhombus Square.
In CrossCode
After Lea escapes from the M.S. Solar, Sergey teleports her to Newcomer's Bridge, right after the second stage of the Rhombus Dungeon. Here, she meets Emilie, who challenges her to a race on the last portion of the dungeon. After the dungeon, Lea is confronted by Apollo, who accuses her of cheating due to her overly high level. Joern soon arrives and defuses the situation. Lea and Emilie then continue through CrossCentral to Rookie Harbor.
After obtaining the Meteor Shade, Lea's friends invite her to come explore Rhombus Square with them, but she can choose to skip this and go directly to the Old Hideout.
Near the end of the game, Lea runs into Albert and Ivan having a chat in Rhombus Square. After realizing who Ivan is, Sergey gets in contact with Albert and arranges a meeting. Lea returns to Rhombus Square and visits Ivan in his mansion, where he subjects her to a Turing test. Details of this are given on the Endings page.
In the A New Home DLC, Lea must pass through Rhombus Square to reach the newly unlocked area, Azure Archipelago. After exploring the archipelago, she returns to Rhombus Square for her fourth duel against Apollo, which takes place in the Open PVP Fields.
Other notable information
CrossCentral provides a teleport hub with links to seven locations, as well as direct access to the main city. The teleport links can only be unlocked from the other side by entering the large landmark teleporters found in the Playground.
CrossCentral is also referred to in-game as CrossStation, although this may be specifically the sky rail terminal located on its roof.
One of the Strange Logs can be found here, in a bookshelf in Old Smith 1F.
One of the Captain's Reports can be found here, in Alabaster Passage on a ledge to the northwest across a road.
In Shopping District, a drifting car playing loud eurobeat music will occasionally pass by on the highway.
Locations of CrossCode
Meteor Shade. Of particular note are a shopping district in the east and the Arena in the west. The Vestorovich Manor is not immediately unlocked, and is only opened during the good ending storyline. Full access to the rest of the city is obtained at the beginning of Chapter 11, once Sergey gives the Full Rhombus Pass to Lea.
Silver
Emerald
Bronze
Golden Ticket
Gold
Normal
Found in secluded garden area in Rhombus Square.
Rise Booster
Skirmisher
Lazy Claw, 2x 
Serene Bubble
Zealous Lamellar
Deadly Dancers
Hydra Fang
Gorgon Veil
Centaur Vest
Unicorn Greaves
Meister Gloves
Lifefiber Uniform
Gourmet Steak
Autumn Leaves, 
Hotsauce
Chef Sandwich
Rising Super Star
Cobalt Goggles
Cobalt Edge
Cobalt Mail
Cobalt Boots
Skirmisher+
Serene Bubble+
Zealous Lamellar+
Deadly Dancers+
Rift Spore, 7x 
Burglar's Rope, 10x 
Ancient Shade
Symbol of Neutral