As others have pointed out, commercial activity is what made people move. There was definitely a dichotomy where the folks that didn't travel never traveled and those that did were almost constantly on the move. Between wars, pilgrimages, and commercial activity its' easy to imagine how the disease could spread.

I focus more on Western Europe during this time, but you are right to say that pitched battles were much less common than sieges. Pitched battles often resulted in the end of a campaign for one side. Whereas strategically sieging castles/cities gave broad command of an area, losing a pitched battle could effectively mean loss of the war as a whole in an instant. Capture of key figures like high nobles or the king himself could be the undoing of an army regardless of their number or quality. Unlike TV/Fantasy portrayals, in a feudal society a leader had to convince vassals to come to war with him. Famously, English kings struggled to get Northumbrian barons (northern England) to go to war in France because they saw very little gain for themselves being so far removed. Even more than just losing simple lives, losing a battle cost a king one of the most vital resources he had: confidence. Powerful nobles would often change sides or abandon the cause, sometimes right before a battle. For these reasons and many more, a methodical campaign of sieges was preferred.

To your question, the first thing a ruler needed was a cause. I think of Plantagenet kings like King John trying to reconquer their familial holdings in France. Kings were known to come up with wild reasons to go to war, many of which were over exaggerations or outright lies. But, there had to be a specific aim and a reason to fight (especially when warring against another Christian nation). The next thing they needed is the money to carry out a campaign. Funding would come from taxes levied on barons (the same Kings depended on to fight with them), the church, and even peasants through royal sheriffs and taxes on trade/products. Some kings ran wild with taxation, like charging Barons insane sums just to inherit their father's land and estates. The funds a king extracted would be needed for supplies, transport, and most importantly, mercenaries. When cash on hand ran out, many kings would borrow from other nations (like Italian bankers) who were basically speculating on the outcome. Many of these bankers lost their lunch when the king they financed lost his war.

The promise of plunder and a regular paycheck was very appealing to mercenary bands, and for that reason medieval nations rarely went to battle with only their own troops. Mercenaries were often times more loyal than nobles but were prone to running wild when the fighting was over. Sometimes they had to be physically driven out by the lords that hired them. Flemish mercenaries were widely hated by the English in particular.

I'm a little less well read on campaign planning as a whole, or maybe we just don't know how it was done in any great detail. Generally, the English would try to capture a castle along the coast that could serve as a hub for further attacks (see Harfleur). Supplies brought from home would only last so long, so it was commonplace for nearby villages to be raided of food and any valuables. These supplies were carried behind armies in what is known as a baggage train. Cart after cart of supplies was often a target for opposing armies, who could defeat an opponent without fighting them in the open by starving him.

Crop burning and general destruction was rampant as well. The English caused especially visceral damage to the French as a dedicated strategy during the Hundred Years' War, partly to try and force a pitched battle. Many losses in any campaign came from disease, especially dysentery (called bloody flux at the time) because siege camps were mostly disgusting places with cramped conditions and very little sanitation.

Victory or defeat was usually cemented by the signing of a treaty, which would have terms that could include the secession of land, money, and even marriages. Usually a country was brought to its knees in a flashpoint, like the capture of a vital position or massive defeat in a pitched battle. I think The Hundred Years War: The English in France 1337-1453 by Desmond Steward is a book you would like. This series of conflicts has fundamental tenets of warfare that I think apply broadly to any ruler going to war in the Middle Ages. Most of my answer has an English slant because that's the only area I feel comfortable answering for.

This is a pretty simplified rambling explanation, but I hope it at least points you in the right direction!

I find it fascinating that the more we actually take ancient writings, maps, drawings, etc. seriously, the more they tend to be remarkably accurate. I think the future of history/archaeology is going to be giving serious inquiry to discoveries that were traditionally written off.

I would argue that learning languages was easier for many people then, especially in centers of trade like a Greek city-state. As the other commenter mentioned, there were an abundance of multilingual people because of the large exposure to different languages.