That's very difficult to answer. The Medieval Period was a time that saw huge changes in the ways battles, campaigns, and wars were fought. How men were recruited could be as simple as local levies, to paid contractors.

However, I will indulge the overly broad nature of this question and give one example that I feel conveys the diversity and overall structure of battlefield tactics in the Middle Ages. What is notable is a relative dearth of battles between 1150 and 1250, with Henry II and Richard I fighting many campaigns but few pitched battles. The reason for this is complicated.

The battle is the Battle of Arsuf, in 1191. The Crusader Army, under King Richard I of England, was on the march to Jaffa, a key step on the road to Jerusalem. The key source for this is the biased source Itinerarium Regis Ricardi, written by Richard de Templo. I haven't yet read an accompanying account from the Ayyubid side, so any contradictions in their texts would be appreciated.

Saladin attacked the Crusader Army while it was on the march, using a screen of light cavalry archers and dismounted skirmishers to harass and injure the enemy. Behind this were the 'shock force' of the Ayyubid forces, their heavy cavalry from Cairo, Damascus, aswell as Saladin's personal slave-soldiers, the Mamelukes. The intent was to scare, tire, and wound the Crusading forces until they could be decisively attacked and routed by the heavy elements of Saladin's force. Once the Crusader Army had devolved into mass rout, the more dangerous elements could be surrounded and destroyed at will. This shows a clear understanding on Saladin's part of the most efficient way to destroy Crusader forces; weaken, rout, surround, destroy. The wide variety in the discipline of Richard's army was one of his most significant weaknesses.

The response from the Crusader forces was three-fold; keep moving, keep tight, keep shooting. Using crossbowmen as a response to the mounted skirmishers of the Ayyubids, the main job of the nobility was to keep discipline and keep moving, so as not to be surrounded and worn down in the sun. This was accomplished by the spreading out of several highly respected knightly contingents, such as the Knights Hospitaller, who held the rearguard, and the Knights Templar, who formed the vanguard. Once the Ayyubid impetus was spent, the Norman, English, and French contingent could be used as a battle-winning shock instrument. Essentially, the two commanders both had similar strategies; however, Richard's ability to maintain the tactical defensive meant that Saladin was forced to engage elements of the Crusader force in melee. Many Crusader horses were killed, but their riders helped to stiffen the infantry of the column.

The critical moment came when a contingent of Hospitaller crossbowmen broke ranks and began to be cut up by the heavy cavalry of the Ayyubid forces. In the process of this, a significant portion of Saladin's army lost discipline in its enthusiasm; though they remained tightly formed, they could no longer respond effectively to changes on the battlefield. This was to prove their downfall. The English King maintained his 'standfast' order via the use of trumpets In response, the Hospitaller and French contingents disobeyed the command of Richard and counter-charged the Ayyubid forces.

I want you to understand that this is one of the most terrifying things any man in Europe or Asia could face at this time, because many of these men were born to do this. The French and Hospitaller knights were made up primarily of noblemen who had been raised to fight, kill, ride, and win from the age of 8. Many of them had the very best equipment and mounts money could buy, and they were battle-hardened with faith, fervour, and experience.

Richard saw that if a general counter-attack was not called, even these experienced troops would be surrounded and killed. So, he adapted. Throwing his mounted knights into the fray across the line, the Crusader army crumpled up the tightly-packed ranks of the Ayyubids which had been harassing their rear, and the sea-change from defense to offense caught Saladin off-guard. With a significant portion of the army routing from the sheer impact of Richard's improvised counterattack, the Ayyubid forces routed.

A second charge against regrouped Ayyubid forces ended the battle, with many overeager knights being cut down as they overextended and were turned on by their quick, nimble opponents.

I hope this goes some way to showing the battle tactics of two highly competent medieval generals. Saladin's plan was clever, and would likely have succeeded against a more cautious or less confident commander. In return, Richard reacted brilliantly to events, having a clear counter strategy in his head that he adapted to achieve victory. The Crusader Army found its weaknesses pressed, but were able to better utilise their strengths via luck, timing, and determination.