The logical place to look first is the actual D&D rulebooks, so that's where we'll go next.
Although I have posted in the past about adapting their sample dungeon material to Mystara, the first two Basic Sets (Eric Holmes' and Tom Moldvay's editions) contain no reference at all to the Known World. They arguably contain no mention of a setting as we define it now at all, beyond the scope of "There's this town called portown and there's a dungeon nearby".
When we get to examining the Karameikos Gazetteer sourcebook, it becomes apparent that the sample Group Dungeon in the DM's book from Frank Mentzer's Basic Set has been retro-assumed to be set in Karameikos, in the town of Threshold and the nearby ruins, but a careful reading of the rule book on simply its own content reveals no such tie to the setting. The PCs are in a town, there's a ruin called Mystamere Castle nearby where the evil icky-bad villain Bargle is hiding out. That's it.
The Rules Cyclopedia contains a sort of mini-primer on the Mystara campaign world, published after the bulk of the Known World Gazetteers and at roughly the same time as the setting was being prepared for conversion into the AD&D 2nd edition world of Mystara, so we will come back to it later. The various "Basic Sets" that followed the RC return to the early trend of not assuming or describing any larger setting for their sample dungeon.
That brings us to the Expert Sets. Starting with Dave Cook's edition. The sample wilderness key and map section has some information.
...the map shows a section of the Grand Duchy of Karameikos. The Duchy is a large tract of wilderness and unsettled land claimed by Duke Stefan Karameikos the Third. Although he claims control of a large area of land on paper, large portions of it are held by humanoids and monsters. The two main settled areas are the coast near the main city of Specularum and the Black Eagle Barony on the Gulf of Halag.Two new bits here. First, Duke Stefan is "the third". This part of his title was not mentioned in the Isle of Dread material. Second is the introduction of the Black Eagle Barony and the naming of the large bay/gulf between Karameikos and the Five Shires as the Gulf of Halag.
The weather throughout the area represented on this map is generally temperate and mild with short winters of little or no snowfall and long summers. Rainfall is ample but not heavy and easterly winds blow cool breezes from over the sea.
The mountain range running along the north edge of the map is known by different names by the peoples of the territory including the Black Peaks, the Cruth Mountains, or The Steach. The two large river systems that provide drainage from the area are left for the DM to name.
I really like that second part. While canon, including the maps from Isle of Dread, calls those peaks the Cruth Mountains, the locals also call them the Black Peaks or The Steach. These are great imagination joggers for a DM wanting to expand things. What evils lurk in the mountains that gave rise to these other names?
Due to the climate, large sections of this map are heavily forested. Humans engage in lumber operations near the edges of the forests, but are loathe to venture too deeply without good cause. Timber, both hardwood and softwood, is a prime resource of the area, and is either exported or used to build ships in the shipyards of the port of Specularum.Again, the hint of mystery and danger lurking in the depths of the forests. While the map shows some of the major monster types to be found in various regions of Karameikos' forests, literally anything could be waiting for those adventurers brave or foolish enough to wander off the loggers' trails.
Without dictating too much of the local flavor, this gives us a general idea of what Specularum is all about. It's a maritime trading city with a strong military presence. Safe and secure. I quibble with the low population a little, but this is one case where later material (the Karameikos Gaz, in fact) fixes that, bumping it up to 50,000.
Specularum - Originally a trading port founded when this area was first explored, Specularum has become the major city of the Grand Duchy of Karameikos. Approximately 5,000 people live in or immediately around the city. The Duke maintains a standing force of 500 troops and may raise an army of 5,000 from the surrounding countryside in times of war. A small fleet of warships is maintained in the harbor.
The city is primarily noted for its excellent harbor facilities and shipyards. Walled on the landward side, the city is also protected by 2 breakwaters that extend into the harbor, restricting passage to a narrow entrance. Overlooking the harbor is the Duke's castle, providing ample defense of the harbor.
Black Eagle Barony - This area of the Duchy has been given as a fiefdom to Baron Ludwig "Black Eagle" von Hendriks. The central town is Fort Doom, a forbidding structure. It is rumored to have dungeons filled with those who have displeased the Baron, an extremely cruel and unpopular man. The Baron may have possible connections with evil slavers and disreputable mercenaries. The Baron maintains a garrison of 200 troops, using them freely to quell dissent and cruch attacking non-humans.Again, without force feeding us a bunch of detail, this paragraph manages to conjure up quite a few ideas for adventure. Rescuing allies from the Baron's dungeons, fighting off the slavers (or being captured by them and forced to escape!), run ins with corrupt troops or mercenaries. Fun stuff.
Luln - Composed primarily of persons who have fled Black Eagle Barony, merchants who have come to trade with the Baron, and some non-humans who have left the wilderness, Luln is a base town for adventurers esploring the Haunted Keep, also called Koriszegy Keep, and the surrounding land. Somewhat lawless and open, the town can provide most of the basic needs to any group of adventurers. The town is poorly defended, relying on the goodwill and capabilities of both the Baron and the Duke for its defense. Approximately 500 people live in the town.Although later material devoted to Karameikos shifted the presumed base of operations of the PCs to Threshold, this entry shows that Luln is an ideal spot to center a campaign around. Lots of adventure opportunities abound nearby, and the possibility of local adventurers being called upon to defend the town until reinforcements from the Baron or Duke arrive.
The text goes on to add a little detail on the gnomes of the Duchy, including a small sample dungeon based in a typical gnome stronghold. This stuff isn't especially relevant to a larger examination of the setting, so I'm not going to reprint it here.
Lastly, a fairly small scale, detail map of Karameikos is included. The most useful part of this map are tags detailing what types of monsters are found in the various regions. The area that they chose to place the keep on the borderlands module in is infested with Frost Giants! Watch out!