Altearth is our world with a significant deviation: Rome did not fall to the barbarians; Rome fell to goblins.
Assume that it was goblins and trolls and kobolds who poured into the West in the 5th and 6th centuries. Assume that as that happened, magic became a real and genuine force. Assume that all the monsters of fantasy from that point onward became real. And, finally, as far as possible, assume that all other historical events continued to happen. That is, there was still a Charlemagne, still a Hundred Years War, and so on.
The rest is just working out the details, though some of these are crucial. For example, while Christianity exists in Altearth, it is only an obscure cult. That means no Roman Catholic Church to dominate society, no Crusades, and so on. But, that leaves room now for the Great Houses of Magic to dominate society, and for an epic war not against the Infidel but against the drow. Ringing the changes on such postulates is where I find my primary amusement and forms the bulk of what you will find on this site.
As for the pictures, I try to find real photographs that are either so old or so exotic that they can be passed off as pictures of Altearth. In many cases I have treated the picture in some way to make it look more “real”. I remove stray telephone poles, for example. I shave off crosses or other obvious religious references. I color-tint the black-and-white pictures, or sometimes I convert color pictures to monochrome. It’s essentially the same sort of playing with images that I am also doing with historical events.
I end the history of Altearth in 23rd century for the simple reason that my detailed knowledge of Earth history ends in the 17th century. Also, once the New World is discovered, I found it greatly complicated the creation of alternate history, so it was easy to stop there.
Finally, a word on dates. All dates in Altearth are ab urbe condite — from the founding of the city, which is of course the founding of Rome. The Earth dating for the traditional founding is 753BC. So, to find an equivalent Earth date for any Altearth date, subtract 753. For those of you who care, the Gregorian calendar reform did indeed happen in Altearth, for pretty much the same reasons. Not Church-sponsored, of course, but since it was developed by the House of the Red Hand, the other Hands adopted it somewhat haphazardly over the next century. Britain was the last to do so.
Origin in more detail
I first invented Altearth when I was playing GURPS with a group of friends. I wanted to run a game and every time I came up with ideas, I found myself plundering my knowledge of history. So one day in 1995 I decided to quit trying to hide the fact and just use my knowledge of European history directly.
Two ideas were key. The first was the name: Altearth. I liked the name because it was an alternate reality, but also “alt” means “old” in German, so it worked in two senses. Plus, being a computer geek, I liked the idea that my invented world was like the real world, but with the Alt key held down. If you get the perfect name for something, you really are obliged to explore it.
At virtually the same time, I invented the key postulate of Altearth: that it was an alternate path of our own reality. Thus, classical history is the same in both worlds, with very slight differences. But in the later Roman Empire, it was not the barbarians who invaded, it was monsters. Some time in the 3rd or 4th centuries, Altearth diverges from real Earth. Magic is real, monsters are real, and the Roman Empire reeled under troll hammers, orc spears, and goblinfire.
A simple corollary was that the various strange and miraculous stories from the Middle Ages really did happen. This lets me have fun being literal, so that Henry the Lion–a very real Saxon duke–can become a werelion in Altearth.
I did not quite know what to do with Christianity, so I simply ignored it. Christians exist in Altearth, but they are a very minor cult of no historical significance. But I very roughly substitute the Great Houses and the influence and role of magicians for the Catholic Church and the influence and role of priests. Monasteries and cathedrals become Great Houses and Charter Houses. As with everything else in Altearth, this material is malleable and the reader is likely to spot inconsistencies throughout.
Awkward for me is the handling of the “bad guys” in a variety of contexts. In the Hundred Years’ War, for example, I chose to let the drow and their allies win most of the battles, regardless of whether they were won by the French or the English in real Earth. But the bad guys have to come from somewhere, and inevitably this leads to me portraying some specific place (e.g. Scotland) as being “bad” place. I hope this offends no one, but I imagine that’s too much to hope. The most delicate matter in this regard is handling the Near East. I have consciously not had any sort of movement of bad guys come out of the Arabian peninsula in the 7th century, but have chosen instead to let them originate from a variety of sources around that time. As my focus is on western Europe, this has led me to make eastern Europe, the Near East, and North Africa serve as distant sources for looming threats. I suppose if I were a Byzantine historian, I would have France and Germany be the scary places.
The basic technique for working in Altearth is to take as much of actual history as is convenient, consistent, and convincing, mix in your standard D&D stuff, and let simmer. Rather than trying to write stories, I’ve chosen to employ a variety of literary forms, including everything from scholarly essays to sonnets.