Frengong is the true name of the buried Elderling city under the Rain Wilds city Trehaug. Though for the most part referred to as the "buried Elderling city," it is Tintaglia the dragon who recalls its true name, having visited the city in her memories.
The Elderlings did not build their cities in series of houses on streets, as a regular city would be built. They built their cities like honeycombes, with all the rooms in the house (the chambers) connected by corridors. These chambers could then be connected by further corridors to other houses, forming large communal houses.
Frengong was destroyed by the Great Quake, and the Crowned Rooster Chamber was used to protect the Dragons' cocoons during the cataclysm.
Frengong would have had all the classic features of Elderling architecture, including wide and shallow steps, a large cleared space, extremely high ceilings, and wide streets, all to accommodate the Dragons that they shared their world with.
The main chamber in Frengong was the Crowned Rooster Chamber. It was an immense chamber topped with a high, glass dome. The chamber was used for gatherings, meetings, and for the carving of the memory stones. There were doors and mechanisms built to accomodate the large pieces of memory stone that would have been brought into the chamber for the carving of "Elderling Dragons."
There was also a large theatre, as the Elderlings were a cultured people. There were further corridors to allow for the actors to dress, rehearse, and write their performances. There were also doors and trapdoors for the actors to access the stage, unseen to the audience.
In Malta Vestrit's "memories" of Frengong, she recalled that there was a boat dock and a wide veranda at the back (or possibly to the side) of the theatre. When Frengong was still thriving, the river was not as corrosive as it is now, allowing for sturdy boats to float on the river.
After the destruction of Frengong, the dock was at ground level, suggesting that the land around Frengong rose after the Great Quake. There is further evidence of this in the complete burial of the Crowned Rooster Chamber after the disaster, including its high dome.