There never was a Tlecaxana Empire as such; only a large area settled and developed by a network of city-states united (and occasionally divided) by trade, religion, and (mostly) similar social structure. Most cities were aristocratic republics, with the dominant ideology prescribing a class of helots working the outlying land, a class of sorcerous-blooded warriors enjoying a franchise (used to vote for a noble subset), and a class of priests producing high culture and staffing permanent bureaucracies. However, this caste system was loose and informal, with people moving between groups not infrequently as well as additional categories not fitting into the normative three-fold distinction – personal slaves, peripatetic merchants, people (working, fighting, administering) but not socially in the “correct” caste, ritually untouchable people, and a growing group of urban laborers. And although in theory the helots were bound to cities, in practice particular warrior-nobilary families had and could trade rights to their produce (although these rights could not usually be sold to those outside the city.)
At the time of the Ulfen invasion, a group called the League of Four (after the four cities constituting the core of the alliance) was attempting to unite the Tlecaxanas in to a true empire; led by the dread archmage Tatlanco and her puppet kings, the League won quick military victories along with universal enmity by allying itself with the powers of the Abyss. When the Ulfen came with magics previously unknown to Arcadia, they found ready allies against the demon state, and rapidly established control.
The boundaries between the Tlecaxana and Tulita are mostly politics and economics: the Tlecaxana never developed very advanced naval technology, and so found policing the archipelago along the Razor Coast difficult, and so for centuries, helots fleeing for the southern coast would join the Tulita, while captured Tulita would be enserfed as Tlecaxana helots.