Theory: The Tribe of Ancients Built the TTYD Ancient City

The ancient civilization that built and inhabited the ancient city from Paper Mario: The Thousand-Year Door was the Tribe of Ancients from Super Paper Mario.

Arguments

 * Argument 1: Many symbols and architectural features are shared between the ancient city and structures built by the Ancients. First of all, Fort Francis, originally a dwelling of the Ancients, and Castle Bleck share similar conical roofs and the same fortress-like design with the ancient city, with high towers and tall walls. The houses seen in Yold Town, an old settlement of the Ancients, look similar to the houses seen in the background of Rogueport Sewers, with blocky roofs and small square windows. The Riddle Tower also looks very similar to the temple Merlumina resides, with its deep blue walls and its tall blue pillars. The most striking similarity between the two are the symbols. The Ancients mostly use symbols resembling plants, as seen on many places in Flipside and the back wall of the temple Merlumina rests. There is one particular symbol that looks nearly identical to one found in the ancient city; a symbol looking like two intertwining branches can be seen on the walls in Rogueport Sewers, on fences in the Palace of Shadow and on the base of the fences in Flipside. The presence of symbols involving branches and flowers is also very common in the Palace of Shadow, with the rose featured heavily, as well as on some decor in Flipside


 * Argument 2: A handful of Shamans live in Rogueport. These Shamans are Merlon, Merluvlee, Merlee and Chet Rippo, who are all almost definitely descendants of the Tribe of Ancients. Wonky says that these four are all part of a strange tribe that names members by their profession, emphasizing that this means that there are many other Merlons in the world besides the one in Rogueport, giving an explanation for Shaman characters bearing the same names across the Paper Mario franchise. This also means they’re all descendants of the Tribe of Ancients. Shaman characters aren’t often found and only show up in important locations associated with the Ancients, or in more desolate areas. Seeing as Rogueport is well-populated, this suggests that the location bears some significant importance to the Shamans.


 * Argument 3: According to Super Paper Mario’s backstory, the Tribe of Ancients’ members spread to various worlds to pass the rest of their days after the Pure Hearts were scattered. Since we see shamans in many places in Mario’s home dimension, at least some Ancients went there once they scattered and one of these places could very well be the ancient city. Seeing as the ancient city was destroyed 1000 years ago and had prospered before that, Ancients establishing the city 1500 years ago would match up very well, as the city would have a good 500 years to expand and gain notoriety before being destroyed.


 * Argument 4: Both the Tribe of Ancients and the ancient civilization that inhabited the ancient city put great importance on the stars. Shamans wear brooches or earrings bearing stars, they read the stars to predict the future, they care for Star Children in the Mushroom Kingdom and stars are often depicted in their architecture. The ancient city features star patterns, a giant star statue is seen in front of the Riddle Tower, and the inside walls of the tower are decorated with stars. But most important is the magic used to create the crystal stars. The Crystal Stars are of course in the shape of stars, but they were created by the Shadow Queen using the ‘light of the stars descended from above’.


 * Argument 5: Both the seal seen whenever a Crystal Star is lifted in front of the Thousand-Year Door and the pattern seen within Count Bleck’s cape when he’s performing magic share the same texture, implying that both are similar kinds of magic.

"You've heard about the legendary treasure hidden beneath Rogueport, right? Apparently it's the vast fortune of that ancient, forgotten civilization..."
 * Argument 6: (A series of quotes from The Thousand-Year Door that link the two civilizations)

- Wonky


 * (The Tribe of Ancients fits this description perfectly)

"You really feel the grandness of that ancient civilization when you're down here..."

- Goombella


 * (The Tribe of Ancients was powerful and grand)

"Oh Master of darkness and shadow… who knows the secrets of the stars… The stones with the power of the stars… house your powers as well… we, the great ones… who know the secrets of the heavens… know that all seven were created… from the light of the stars descended from above."

- Riddle Tower


 * (A compilation of the text seen in the Riddle Tower describing the creation of the Crystal Stars, as translated directly from Japanese. This text would fit perfectly with the Ancients, as they know much about the ‘secrets of the heavens’, as they used the stars to make predictions. The Great ones also fits well with the Ancients, as they and the Tribe of Darkness are portrayed as very prideful)