I am always fascinated by ancient history. There is so much that is intriguing, and damned cool. Of course, I liberally use such inspirations for my own campaign world of Thandor. Ancient Greece, Ancient Rome, Babylon, India, the Chinese Empire, the Celts, and on and on. Just a lot of really great stuff to be found in studying these kinds of sources about cultures, history and civilizations.
(1) The medieval Vikings had a forging technique--pattern welding--that allowed them to make swords that were flexible, strong, and durable, and entirely superior to anything else in Europe. Rough equivalent knowledge were in Toledo, Spain, and in Damascus, Syria. However, it should be noted, the famous Japanese Samurai swords--the technique they used--was not notably superior to the Viking technology. From what I understand--though I am no swordsmith--the Viking technique arrived at a similar place in swordsmithing as the Japanese techniques, though merely by a different path.
(2) The Norsemen were great explorers, and magnificent sailors. Indeed, the Vikings discovered North America long before Christopher Columbus, and had established a settlement that endured for some time in Newfoundland. The Vikings buried their dead there, and in addition, other Viking artifacts have been found, which combined, cements the achievement that the Vikings settled North America 500 years or more before Columbus. Interestingly, some native folklore suggests that various Indian tribes not only encountered the Vikings, but may also have traded with them, waged war, and adopted them into the tribes or otherwise intermarried with individual Vikings.
(1) The Romans developed a variety of welding techniques--some of which were not matched until after the 17th and even 19th centuries. The Romans mastered engineering to such an extent as to create a civilization that enjoyed many elements of what we know of modern society--though the Romans achieved this more than 1500 years ago. The Romans had public bathhouses; plumbing; hot and cold water; heating and air conditioning; vast apartment buildings; shopping malls; fast food; public laundry-mats; daily news services; Public bulletin boards; Hospitals; Schools; Libraries; Brothels; Guilds/Unions; Organized Crime; Political *campaigns*; voting; scandals; and a political climate not far removed in substance from modern day America or Britain; Not to mention advanced medicine and medical practices; the development and spread of *books* and *literature*; and literacy rates for the general population equal to America in the early 20th century.
The Romans also of course possessed advanced military skills, vast armies, a superb navy; special forces. In addition, the Romans had developed their famous roads that connected thousands of miles of the empire, and made swift travel possible; as well as aquaducts that brought plentiful water to every major city and town; a empire-wide mail service; and an advanced sea-going merchant network that allowed the timely transport of thousands of tons of food from thousands of miles away--Egypt--to the city of Rome, so as to feed more than one million inhabitants. These networks also allowed the feeding of other enormous cities in the same network--and thus enhance and increase the populations of many cities to vast heights, and thereby also expand trade to never before seen proportions.
It's no wonder why the Roman Empire captures the imagination throughout the centuries since the fall of Rome. The Romans really did possess a huge, advanced civilization that absolutely overwhelmed and stunned all surrounding civilizations by a vast margin. The Roman Empire boggled the imaginations of everyone--truth be told, the Romans were, much like modern people--also staggered and in awe of themselves as well, to such an extent that the Roman civilization was so vast and so advanced and complex, there were many secrets and techniques that remained a mystery to many normal Romans. (Much like automobiles, computers, and other technologies boggle most people except for educated and trained professionals). Perhaps most interesting--every few years or so, archaeologists and historians alike uncover new details and glimpses that demonstrate that the Romans were even more advanced than previously believed, either in some technique, breadth, or scope of achievement and knowledge--the Romans continue to surprise modern scholars with new heights of Roman civilization.
Just a few tidbits to inspire from the ancient world.