Saronia is one of the three big kingdoms of the world and the only one remaining on the surface. Of the three, Saronia is the biggest. While castle towns are not new to the series (Cornelia, Elfland and Finn all had towns in front of them) nor would it be the last, Saronia stands out due to its city being split into four districts. This size is further reflected in the name of the city's unique theme: "The Megalopolis of Saronia".
Saronia's history in-game is kept to a few minor implications. A lot of the city's history comes from the NTT publishing guidebooks, only roughly half of which have been scanned at this point. Thus, for now, the NTT stuff is largely hearsay, come from secondary sources.
According to said NTT guidebooks, Saronia once went to war with Sasune, an event known as "The Sasune-Saronia War". This event gave rise to specialized magic, with Saronia creating White Magic to counter Sasune's black magic.
While this worldbuilding aspect is not included in the remake, there are still minor indications that Saronia used to be a more militant country that invaded its neighbors for power. Most notably, an NPC mentions that Saronia once sought out Falgabard for the power of its Dark Blades and outright says that Saronia used to be a military nation. Another NPC mentions that Saronia used to have the greatest military in the world due to their airships and dragoons.
In the remake, Cid Haze is implied to have come from Saronia, and was in the process of resurrecting the Nautilus from the nearby Ancient Ruin.
The Northwestern District is home to the Great Library of Saronia: A library that touts to have every book ever written. The library is closed during the civil war, but after the Warriors of Light save the kingdom, they can go in and read some of the books. The info in the books is largely foreshadowing for later locations, but the Ancients Book and Book of Owen give some worldbuilding crumbs.
In unused text for the remake, Alus notes that the district is full of scholars, engineers, lawyers and people in politics and that those who played an important role in royal family make their residence there. In the final game, the Warriors of Light first meet the Legendary Smith here.
It is not known if there is any particular lore from the NTT guidebook regarding its backstory, hinting that this district may have been one of the earlier ones.
The Southwestern District' point of interest is a tavern, where the Warriors of Light initially encounter Prince Alus Restor.. Alus mentions that the tavern used to see good business "in the olden days". The district also is the home of a Chocobo researcher, who can call the Fat Chocobo for the party for free.
Of the four districts, the Southwestern one is probably the least notable. The only reason to visit the tavern is to meet Alus and there is nothing to do after that (aside from the free Fat Chocobo).
Interestingly, the remake has a copy of the dialogue after the party's airship crashes. Whether the airship was supposed to crash over this district (thus putting the player close to where Alus is) or for some other reason is unclear, but it's worth mentioning that the other districts do not have it.
The NTT Publishing guide supposedly states that the city was once home to a clan of Knights, opposite the Northeastern District.
The Northeastern District of Saronia is said to be the city's economic center by Alus in the remake. It hosts a slew of shops, but during the civil war, the king ordered them all to close their doors. However, one shop (conveniently selling Dragoon equipment) defied the king's orders and remained open.
The NTT publishing guide supposedly gives a surprisingly complex backstory for the district: It and the Southwestern district were the primary residences of two clans of knights: The Light-centric Knights and the Dark-centric Dark Knights/Warmages.That changed when the king of Saronia came to fear the power of the Dark Knights. Thus, he began banishing Dark Knights, starting with one that just so happened to be named "Leonhart". The Knights would come to inhabit this district as well while Leonhart would retreat to the mountains and his fellow Dark Knights would follow him, eventually settling a covert village...
The Southeastern District is where the Dragon Spire stands. The Dragon Spire is a large (though simple on the inside) tower that was built by dragoons. While the dragoons of Saronia are no longer around, their treasure still remains.
The Dragon Spire has its own backstory. Coincidence or not, the version Alus tells in unused text for the remake is very similar to what the NTT Publishing guide (supposedly) had:
はるか昔 サロニアをおそった 巨大な鳥の怪物を 倒した騎士… 彼は竜の背に乗り すばらしい跳躍力で 敵の頭上に急降下する戦法をとりました 竜を御し 凱旋した彼の姿から 竜騎士という称号が生まれたのです ドラゴンの塔には 彼が使っていた武器が 収められているのです
The rough translation: In the past, Saronia was menaced by a bird monster. Said bird monster (heavily implied to have been Garuda himself) was eventually slain by a knight riding a dragon who boasted a powerful leaping ability. This man introduced the concept of Dragoons or Dragon Knights. The NTT Publishing guide (supposedly) claims that there was a clan of knights that would create a manor in the Southeast of the kingdom, which eventually became its own city. This detail is more obscured in the 3D and Pixel Remaster versions, as the Dragon Spire was redesigned to be more akin to a fortress rather than the same as the buildings around Saronia.
The 3D remake has an unused scenario which suggests that the Dragon Spire was originally going to be locked. The party's attempts to get more information on the Dragon Spire would culminate in Arc, annoyed at Ingus and Refia for treating him like a child, leaving the party. The remaining Warriors of Light then must follow Arc and Alus around Saronia as Arc tries to get the key to the Spire. To make a long story short: He does.