A lot of questions about medieval warfare get answered with "well, you know, the middle ages was a long time and europe is a big pace. There's no one answer to your question since medieval warfare changed over time and place."
That makes me curious: what are the major era divisions in the history of medieval european warfare? If you wanted to give a comprehensive answer to a very general question (e.g. "how important were shields" or something similar), how many time periods would you typically divide your answer into?
Like any question about dividing history into different periods, I'm sure there's no perfectly clean choice of divisions. But, I've seen some value in dividing up other kinds of history into time periods, so I assume something similar can be done for medieval warfare.
The most common division of the Middle Ages I see is to divide it into three sections: Early, High, and Late. You can obviously divide it differently and the divisions between the eras will vary some by region. The rough division is:
Early Middle Ages is roughly Fall of Rome (c. 5th/6th century) until 10th century. It includes the last decline of Rome, the establishment of the Carolingian empire, and the vast majority of the Viking raids.
High Middle Ages is roughly 11th century through 13th century. This includes the Crusades as well as several other smaller conflicts.
Late Middle Ages is roughly 14th century through 16th century. This is the Hundred Years War and War of the Roses era.