The Crusades was a very tumultuous time for both the Christian and Muslim factions trying to take the area around Jerusalem. Both forces began the Crusades with tools and tactics better designed for their homelands. Christian foot soldiers were common in their forested homeland but encountered a lack of mobility when countering Muslim horsemen. This led to more horses being used in combat as well as weapons better designed for horseback fighting (lance, halberd, etc.). Anti-horseback weapons also took shape. The hand axe, originally designed to be small and throwable, grew over time, giving rise to great axes that could cleave the legs of a charging mount and pole-arms that could be used to hook an attacker from horseback.
When the Christians entered the war they began wearing mostly chain-mail armor, as it was lightweight and very effective at turning attacks. Muslim defenders however were extremely skilled archers and chainmail provides very poor defense against projectiles. This gave rise to wool or leather surcoats. These were two-fold in being able to be woven with metal discs to make a jerkin or Brigandine armor with added heat protection. Metal rings in mail can get very hot under the desert sun and the surcoat prevented them from burning the wearer. Helms evolved to create better protection, starting as a conical helm and ending as a Great helm which covered every part of the wearer (it was primarily the English who donned them). However, due to the weight of these great helms and their ventilation being as poor as it was, these helms were not worn for long periods of time if it could be helped.
Muslim arms also evolved over time. The average Muslim sword began straight, much like the Christian swords. Over time they evolved into the sabre and scimitars, slightly curved swords with a weighted tip that could swing much faster and thus give an added advantage over the slower swords of the English.
Siege weapons also truly began making their debuts in warfare. Large scale castle assaults became more common, especially when attempting to breach the walls of Jerusalem. Trebuchets, ballista, and other siege weapons were given a reason to be improved and thus the engineering of these weapons was a priority.
There were many ways the Crusades affected weapons and warfare. Armies began to rely much more on horses rather than footmen, resulting in the cavalry replacing the infantry as a primary form of organization. During the Crusades they changed their fighting tactics which changed warfare. There were a lot more weapons during the Crusades that were used including better armor for knights which was later passed down through the west. The armor was much more efficient and lighter.
In warfare, the Crusades really took advantage of castles and forts. They began to construct castles and forts in a different manner that made them a much better form of defense. This was also adopted later when people went to the Holy Land and would go back to western Europe, using the ideas of the castles to create their own structures. Towns and cities also became more fortified so they were stronger against possible attacks.
The best changes of the Crusades came from the improvement of armor, helmets, and shields. Forts, castles, and even towns were built to handle attacks better. Warfare techniques including "siege and prolonged encirclement" were also shown to stem from the Crusades. The improvements caused the wheels to start turning for new weapons and warfare techniques to be improved on even further after the Crusades.