Karadia is a geopolitical (and geohistorical) region that is composed of entire continent of Karathia plus northern parts of Arrica and western of Arzia. It is considered a separate entity due to being a part of a singular superstate (namely Imperium of Karadia) for majority of its history (meaning overwhelming majority of last 3000 years). Which leads to certain similarities and common points in terms of culture, politics, religion and so on.
Karadia exact borderline is a troublesome to point. In the south it ends with the so called Southern Wall Mountains - a range of tall and supposedly haunted mountains. Their existence stabilizes local climate by preventing further expansion of Saratian Desert north, rendering all lands north from it habitable, even if arid and far from being called fertile. In the northeast the border is another mountain range called Eastern Wall Mountains. Which also slightly improves local climate, slightly tampering yearly amplitude of temperature. On the other hand, exact borders here are... debatable. Mountain range itself is controlled by Tyranny of Amner, an expansive semi-totalitarian state extremely hostile towards the Imperium. It doesn't consider itself a part of Karadia (even de iure). Despite this its lands all the way to the eastern border of Eastern Wall Mountains are considered by karadian Grand Emperors as a de iure part of their patrimony - and thus of Karadia. In the southeast the exact border is even more debatable. Imperium of Karadia wages a semi-perpetual war against Kasytian Empire over Akhad, a region that includes both Mezzarian Plains and the Upper Mezzaria. Thus whether they belong to Karadia or to Kasytia is at best debatable.
Northern Arrica includes a mostly arid lands between Southern Wall Mountains and the Native Sea. The situation is vastly different on the coast and, to a point, near the few rivers in the area. There an increased humidity and generally mediterrenean climate allows for an expansive agriculture, focused mostly on traditional crops. With olives and grapes being the most prevalent, although there is also a lot of wheat and gray wheat around. In fact, three regions - khemian lands surrounding river Nihl, Karthian Oligarchy on Karthian Peninsula and a coast between and Cyrenian Peninsula that are a part of personal domain of Grand Emperors - can be considered some of the wealthiest lands in Imperium of Karadia. Which is even further expanded by the free access to most important trade routes in Imperium (or the world itself), meaning the Silk Route through the Iron Channel to the far east, and the Itavian-Vhessan trade route that deals mostly with slaves.
Western Arzia includes mostly mountainous lands of Markarian Peninsula plus Karvathian Upplands and Black Mountains to the northeast from them and Hevarian Coast with Blackpeaks to the south. Mezzarian Plains, as it was said, are in that matter debatable. Hezranian Peninsula is also mostly included in Karadia, although it, as well, is to a point debatable, especially since Hezranian Caliphate started slowly pushing for independence from Imperium. This lands are mostly mountainous and arid, with lots of deserts (especially to the south). The farther north one goes, the more varied the lands are, with big parts of Markarian Peninsula having a lot of woods. To the point of actually becoming its exporter, especially to lands on the northern coast of Black Sea.
Western Karathia includes three major groups of island, namely Vestirian, Alvaenan and Dragarian Isles, and lands of Astaryan Plains and Elvharian Forests. In short - areas mostly covered in woods but also with areas (especially near rivers) that underwent intense settling process. Lots of population, lots of agriculture and industry, plus decent maritime connections. Overwhelming majority of Western Karathia is still covered in woods, especially those on the continent. It is caused by several factors, mostly the fact that Xylian Dominion wasn't really interested in deforestation. With elven culture giving a lot of 'divine' factor to forest, multitude of dryads and lesser wild daemons made extensive deforestation a hard thing to achieve. Such efforts were, ultimately, localized, especially to areas close to rivers or coasts, allowing easy transport without the need to pass through woods. Its land borders are Machine Mountains and White Mountains to the south, Iron Mountains and Odra river to the east (with small exception of lands of Sillian Dukedom to the southwest from it, that is mostly included in Central Karathia due to being ages old part of Visenian crown), and Needle Mountains to the north.
A sort of a curio, since it is mostly composed of three separated peninsulas. Itirrian to the west, Itavian in the middle, and Hellanan to the east. Plus a lot of islands: Batanian Islands, Sera and Setarria, Scilla and Crematorium and all little isles next to Hellanan Peninsula that are too many to count and name. This land takes the most of mediterrenean climate surrounding Native Sea, meaning it is quite dry (but not TOO dry) and hot (but not TOO hot). Lots of olives and wine, substantial population (especially on Itavian Peninsula, which includes central provinces of Imperium), expansive industry and lots of maritime trade. Its easy access to major slave trading routes and to the Iron Channel (which is pretty much an endpoint of Silk Route that connects far eastern countries with Imperium) makes this lands generally quite rich and with many big cities. It is also among the earliest settled areas of the Imperium, which gave it a significant advantage in terms of wealth accumulation (and great architecture).
Central Karathia can be safely divided into two main parts. First to the south and second to the north and northeast. The first one is (mostly) mountainous and heavily forested, with Avareian Lowlands being the sole exception (it still a bit forested though, especially in the southeast). Its northern and eastern borders are the Iron Mountains and the river Duna estuary. South is where Hellana starts, traditionally considered a part of Southern Karathia. The other part is mostly plains, changing into steppes further to the east. It starts with Polenyan Plains to the west, and goes all the way to the Dragonbone Mountains, which today form natural eastern border of Kingdom of Visenia. South from it the border is, more or less, a Don river. Actually a very large part of it are ultimately steppes, that can be (at least regionally) found also in Polenya and other lands north, due to ancient xylian doforestation attempts.
Eastern Karathia is a mostly desolate land where Karadia practically ends quite abruptly. It includes lands between Dragonbone Mountains and Eastern Wall Mountains. From the decimated and ruined lands of former Kingdom of Lomar to the north, to the northern border of Vhessan Empire. Today it is mostly divided between two warring states, a Diretian Freedom to the west and Tyranny of Amner to the east. With everpresent threat of infinitely evil and malevolent beings from north (Nythrathril and his armies of evil from ruins of Lomar plus ice elfs from Ice Kingdoms). Also, a daemons of Chaos, a worse part of Diretian religion, form a permanent threat to everything that lives on Diretian Plains. Plus there is also a permanent threat from Amner, a semi-totalitarian and expansive regime, not beneath relying on help from a very malevolent supernatural beings. Not to mention local climate (cold, dry and almost as far from fertile as it is possible). With long winters (with a lot of snow and very low temperatures), and short summers. Making it quite necessary to use magic (and special plants) to actually support decent population. Overally... it is not a very nice place to live in.
Northern Karathia includes Niflheimian Peninsula and Scavian Peninsula, all the way to the Latakan Lake to the east. It is a curio, since overwhelming majority of it isn't a part of the Imperium of Karadia. And, in thank, never was. Ice Kingdom, a gargantuan decentralized 'country' under theocratic (and ironfisted) rule of ice elves controls majority of them, with sort of border states of Kingdom of Niflheim in the west and Boreal Confederation in the east serving as true border of the Imperium. This region was also heavily decimated by magic during the Great Scavian War. Meaning that most of the Scavian Peninsula is actually almost permanently frozen and would be uninhabitable if not for expansive use of magic and certain crops that doesn't require sunlight or heat to grow (also very magical). Despite this Scavia is considered a part of Karadia by association, mostly because pretty much all other elves beside scavian being part of the Imperium, and the imperial ambitions to expunge the threat one day.