Let's look at why the demons are so well composed.
First the differences between demons - this is double the production of stone ... So what, you say, the rest is also a bonus for resource extraction. But ... compare it with others, to start light elves. Doubling the production of wood gives them 20 basic wood sawmill production and demons 20 rock, a solitary resource, and the stone is then 2 times more expensive! Yes, the elves can build a much larger sawmills, but then there would be no place for laboratories. As a result, the elves have to do some of the city as science, while the demons can easily put all the cities in the mountains, combining resources and mining, and manufacturing science. Now let's deal with the knights, their bonus to trade shops can not even take into account if you do not do a special "golden city", they are carried through too little, and in a city the emphasis is on income from the free population, resulting in no advantage of the Knights exceed 10%. Cereals provide the Knights +50%, is lower than the demons also find steppe level 5 with a lot of cereal fields is not an easy task. And I think the knight would be difficult to live only by selling wheat, making the rest of laboratories. On the dark elves problem is the same - good cells to the metal in the desert is very little, besides metal itself is a specific resource, it is not necessary to start that very difficult initial development.
Conclusion - a bonus for stone is the golden mean, and very successful. On the one hand quarrying would be sufficient to meet the resource requirements, on the other is still a lot of space to build laboratories and production of science.
The second bonus - mercenaries. I must say here and demons do not hurt. When it comes to the ruins of the five - the cost of farming the demons will be minimal. Given that many mercenaries given for quests - they do not even need to build, at their expense can quickly develop.
These combined factors help to reach the level of demons 5 cities faster than other races, though certainly not more important than race character, and its owner.
Good luck in the game, lots of salt lakes and arhiruin!