Some of the most thrilling, memorable moments in fantasy storytelling have been battles. Think of Lord of the Rings' Battle of the Pelennor Fields or Game of Thrones' Battle of the Bastards. Massive armies clashing in a single epic combat that can decide the fate of millions.
Obviously there were no magic or monsters in history. But what were real medieval battles actually like?
Though we think of the era as being technologically static, there was something of an arms race among the states of Europe. For example, the "offense" developed crossbows whose bolts could pierce chain mail armor. The "defense" responded with the invention of plate mail.
Most armies were made up of infantry and cavalry (I suppose you could think of archers as artillery). Don't forget, "infantry" in many cases didn't mean "professional soldier," like the Roman legion. While many nobles and princes had small forces, most foot soldiers in the Middle Ages were regular men, often farmers, that the local lord pressed into duty. They were armed with spears and shields, which they used to form shield walls, which were pretty effective against frontal assaults.
Really, though, cavalry was of critical importance, since it could break through infantry lines, flank formations, and cause general chaos. A mounted soldier, with armor and training and good weapons, could do terrible damage to an infantry company of militiamen. In fact, in many medieval battles, more soldiers were killed during the retreat than in battle. Without a shield wall or a line of spears protecting the infantry, mounted knights could ride through the retreating troops and cut them down with ease.
Cavalry included knights (the best of the best), heavy cavalry (armed with lances), and light cavalry (armed with bows or crossbows). Without cavalry, a battle could take forever. Two lines of infantry with spears and shields would either stalemate or just hack at each other until one side gave up.
Archers were important as well. As good as the crossbow was, the longbow was the dominant weapon on the battlefield until gunpowder came along. Longbows were harder to shoot than crossbows, but their range was far greater, and their lethality was devastating. This is best illustrated in the Battle of Crécy in 1346. The English forces, outnumbered almost three to one, were able to decisively defeat every advance by French troops with a constant hail of deadly arrows.
As seen several times in Game of Thrones, commanders of the two armies (who were sometimes royal or noble) would often meet before the battle and engage in some last-minute diplomacy, trying to convince the other side of the hopelessness of their cause, thus preventing bloodshed.
Armies varied in size. The Battle of Lake Storsjön, in Sweden in 1178, had only about 1,200 men total. The Battle of Bosworth Field in 1485, when Henry Tudor defeated Richard III, saw about 20,000 men on each side, which was around the average during the period. For comparison, the Battle of Gettysburg in 1863 included 104,000 Union troops versus 75,000 Confederates.
I've mentioned how a lot of soldiers were fighting on behalf of their lord. That not as compelling as defending your nation or fighting for a concept like "freedom." Had their lord been defeated, the victorious lord probably wouldn't have affected their lives too much. So why risk your life just so your liege wants more land? Well, that's the feudal system for you.
Also, by the early 12th century, there were organized mercenary companies for hire. While these were able, experienced soldiers, they weren't always emotionally involved in the outcome of battle. Going back to Crécy, the French had Genoese mercenaries attack first; when the arrows rained down, they retreated... and the French then attacked them for their betrayal.
In fact, it was mercenary companies in Italy who developed a new kind of strategic thinking, in order to prevent unprofitable killing. They began to target supply chains, weakening the opponent's economy to make it harder to continue the fight. These companies placed an emphasis on maneuvers, trying to push the opponent's army into a corner where it would have to choose between a suicidal assault or surrender. You don't see much of that clever strategy on screen, when giant armies are more interesting than bloodless maneuvers.
One more way that battles were unfair—knights were often nobles, and nobles usually refrained from killing each other. It was more effective to capture them and ransom them for money or strategic concessions. Lower-class foot soldiers... not so much.
So there you have it. It's hard to compete with dragons and orcs and sorcerers when it comes to storytelling, but real-life medieval warfare could be even more dramatic, tragic, and surprising than the best fiction.
What about sieges, like the Battle of Helm's Deep or the Siege of Riverrun? I'll leave that discussion for another day.