Roundcastle was the first map of Steamothic and the only one to be hosted on the desktop (which meant that it was up much more often but also significantly more laggy). It was the most popular map, with a total of around 10 active players and at least 20 players in total, and was also the longest era of the server. It was originally intended to be a survival map, but due to popular demand and various issues with PermissionsEx, it became a creative/op map. This map was heavily built on, both in Creative and Survival.


Roundcastle had the most builds of any other server, due to its popularity and the fact that it was a creative server. Spawn was built by 17454538 (with some modifications by sparkierjules), and was a round, modern-esque castle made of smooth sandstone, spruce planks, and nether fencing. It had a central fountain and surrounding gardens, and a stairway that led to a lower level with a Nether and End portal. In the four towers were the server rules, a list of players and their respective rankings, a courthouse, and an exit with various utilities. 

Directly next to spawn was a large, octagonal arena built by nacnudd. It was composed of sandstone with various patterns, and featured curved netherack pillars alight with fire, a glass top to spectate, and four chests for gear. The arena was intended to function as dual purpose PvP and Mob arena. Later, nacnudd built a house under this arena made of obsidian. It had various mob heads on the walls and various weapons displayed in item frames, as well as a large storage room full of organized chests. 

17454538 also built a huge underground house for himself. It was accessed by pressing a button on a the side of a hill nearby spawn, which would trigger a hidden piston door that led down a passage and into a 100x100 room with four passages, two of which led to empty rooms, one of which led to a redstone lab for testing redstone machines and mechanisms (this was also used by sparkierjules and chuckie_chuckie), and the last of which led to a small room labeled "FUN :D", featuring spawners of every hostile mob in the game. Later, he built a large obsidian sphere in the sky that he originally intended to turn into a town, but eventually abandoned the project. He also built Delgi, a massive castle with various NPCs created using the Citizens plugin. Delgi was intended to be a quest adventure for players but was never finished due to the PermissionsEx crisis. He worked with peanutandlime to build Funland, an area with various player entertainment features, which he constructed a semi-automatic mob arena and an automatic casino that charged $15. 17454538 also built a giant Mech suit near the mall, a jungle temple, and many other structures around the server.

Peanutandlime built Rainbow Sheepy Sherbet Land, a huge, highly-popular town full of massive, woolen sheep statues of every color, but that were also fully furnished and even had a pet sheep of corresponding color. Almost every player on the server owned a sheep, and at Halloween, there was also a costume contest in which the player gave their sheep a Halloween costume, and was judged by nacnudd, the elected op at the time. Costumes included a Witch sheep, a zombie sheep, a gangster sheep, a Mexican sheep, a wolf sheep, a ladybug sheep, a pig sheep, and even a shoop da whoop. Shadowfire5 won the contest with his gangster sheep, followed by 17454538 in 2nd with his zombie sheep, and Fzone1000 in third with his pig sheep, and honorable mention going to EAMAFAD with her Mexican sheep. During the Halloween party, giant wool pumpkins floated over the area.

Fzone1000, one of the later players to join Roundcastle, built his own town, known as the Mushroom Kingdom in the middle of a swamp biome. Together with nacnudd and making use of worldedit, he built a huge, cubic mushroom made of mushroom blocks and with a mycelium floor. Inside, there was a large house for Fzone1000 that had floors with biome themes, and sewers that led to extensive passages, mines (because of a sign that read "I hid some emeralds in the mine" placed by Fzone1000, nacnudd replaced the entire mine wall with emerald ore), and many hostile mob spawners. Although only Fzone1000 actually lived inside of the Mushroom Kingdom, it was very frequently visited due to its proximity to spawn. Weltinator2 built a small house directly outside of the Mushroom Kingdom. 

Allieandfluff contributed some builds to the map. She made a "Broadway House" in a swamp biome, with multicolored wool rooms and featuring rooms for her favorite Broadway characters, and a large iron, lapis, and glass temple near spawn. She also built an underground house of stone, glass, and glowstone.

Roundcastle was lost when the host computer was presumed dead, thus causing the transition to Savaroc and the gradual decline of the server. However, it has since been re-added to the server via the Multiverse plugin.