Isin is the most easterly of the successor states to Baal Hamazi. Cut off from easy access to its nearest neighbors (such as Pnakot to the northwest) by vigorously expanding tribes of nomadic Untash, it maintains a tenuous connection culturally to the former empire through trade routes that skirt the edges of the Lakama jungle, a place that's as dangerous and off ill-repute as dealing with Untash barbarians is. But Isin isn't known for its isolation nearly as much as it is for its radical and bizarre form of government, and it's strange and wondrous architecture.
Unlike most of the successor states, race (hamazin vs. drylander) is not a significant dynamic in Isin. Rather, it is a caste-based system, and the highest caste--the rulers of the city-state--are those who can operate some degree of witchcraft. This is very much the reverse of most places in the civilized world, where witchcraft is highly illegal, heretical, and suspicion of which can be very dangerous to the accused. Many of the leading class are, in fact, expatriates, or the descendents of such, who come here to practice their craft without interference from the Inquisition, or locals with pitchforks and torches, or what-have-you.
This is not to imply that Isin is a haven and refuge for mages. Mages, by their very nature, end up paranoid and often completely insane. Even if they don't, or fight the urges, the culture of the magi in Isin is very cut-throat, Machiavellian and pseudo-anarchic. Magi live at the top of both the geography and the food-chain, and non-magi have little rights. Most are, in fact, imprisoned within Isin. While their lives aren't necessarily directly controlled by the magi, the magi can, at whim, do whatever they please to them. Their only protection is to get "lost" in the vast herd of imprisoned nobodies--similar to the defense a zebra uses against a lion, by "disappearing" into the herd. Someone's always the loser, but the majority of individuals can survive by being relatively anonymous, or useful to the magi.
But on paper, Agun Kidinu, a hamazin wizard of no small repute, is the leader of the city. In theory, if he tired of the anarchy, he could control it. In reality, it isn't something he's particularly concerned about. There are persistent rumors that secretly Kidinu is one of the Heresiarchy, or if not, a puppet of one of the Heresiarchy... but these remain only rumors.
Below the magi, Kinnad Harfargr rules the non-magi as a dictator. He also prefers a more laissez-faire approach to government, but as resources are scarce and the ability to leave Isin is often limited, the reality for most undercity dwellers is that they are governed by a completely unofficial warlord or gang-leader. Harfargr has access to a powerful force of secret police, he doesn't care to use it except to enforce the will of the magi, or to protect his own power. Frankly, the doings of the undercity dwellers are below his notice, for the most part. Rather, he's concerned with protecting his power and protecting the favor of the magi, since if he loses the latter, he can be easily and fatally replaced.
The architecture of Isin is one of its most striking traits, however. Built inside a vast, vertical sinkhole or ancient lava tube that's nearly a mile across, the city itself clings to the nearly vertical side walls. The magi and their most trusted cohorts live at the top; the farther down you go, the more dreary and anarchic society becomes, as well as more difficult to survive, as it is blighted by gangs and horrors left over or thrown out by experimentations of the magi. Both the magi and the non-magical dictator don't much care what happens at the very bottom of the gigantic sinkhole, unless it threatens to disrupt life above.
Around the upper edge of the city is a massive stone wall, pierced by many gates, but jealously protected by the secret police and the administrative talents of Kinnad Harfargr. The magi are very careful about who they allow to enter Isin, and even more careful about who they allow to leave.
Most of the people living in Isin, as in the rest of the Baal Hamazi lands, are drylanders, but expatriates of various stripes can be found here at all times. One unusual item of note is that inhabitants of the Forbidden Lands or the Plateau of Leng are more likely to be encountered here than anywhere else, as the magi often send expeditions deep into the Lakama Jungles (and beyond) to find them and grill them for knowledge of their blasted and bizarre landscape, which can be converted (sometimes) into arcana power. Sometimes these inhabitants become trusted accomplices of the magi--and sometimes they even enter that caste themselves--but many other times they are simply cast below into the undercity, where they frequently wreak horror on the already oppressed inhabitants thereof.