Note: This is a repost from when I answered a similar question here.

Oh boy, this is going to take some time, and some writing. I will need to post it several sections.

Gustav II Adolf (as he is called in Sweden, Gustavus Adolphus was his latin name) did bring several major innovations to the art of war.

Organisation, equipment and recruitment.

Gustav II Adolf's grandfather, Gustav Eriksson (Vasa) created a set of national volunteer units, and Gustav II Adolf built on this system, creating a regiment in each county (or in case of densely populated counties, several, or sparsely populated counties, several counties per regiment). The system was formally voted into law by the estates parliament 1634, two years after the King's death. The regiment was both an administrative unit and a tactical unit - even if only about 10% of the manpower in the Swedish forces in Germany during the 30 years' war were Swedish or Finnish in origin, the regiments created a formal structure into which mercenaries were sorted - to use Swedish organisation and tactics. This was a break with the former system, where mercenaries were recruited as units and created the embryo of a national army. The regiment called up all men between 15 and 50, and free-holding farmers were divided into groups of 10, and each were to supply a soldier - often a second son to a farmer, a crofter or other man that did not own land - initially, most were volunteers, but as the 30 years' war progressed and the population noted how few returned from the war, more and more coercion had to be used to get soldiers for the regiments.

Gustav II Adolf also created a national arms industry, ordering three factories to be set up at Jönköping, Söderhamn and Norrtälje (the latter two got their city rights at the same time). 1622 another factory was established and Örebro, and 1624 one at Norrköping under the leadership of Louis de Geer, who was placed as a leader of all factories 1627. This process was slow and the authorities had problems getting the skilled artisans to move to the factories, regardless of this however, this created an industry that would produce standardised arms, especially muskets and pistols and high-quality gunpowder for the army. The Swedish army could thus often rely on arms that were if not interchangable at least very similar, which streamlined reload times, creating prepared gunpowder charges and thus improved the rate of fire and decreased the amount of misfires.

Swedish cannon production, with the help of artisan immigrants from Wallonia helping Swedish mining and metal industry, became one of the foremost of the world. Swedish cannons were highly regarded - it can be noted that several of the guns from Blackbeard's Queen Anne's Revenge had been cast in Sweden.

Swedish cavalry horses were small (hardly much larger than ponies - 150cm was a common height for a Swedish cavalry horses of the era), ragged, hardy and easy to train to not panic in the smoke, sound and movement of the battlefield. While Swedish noblemen were often ashamed of the low stature and small size of their horses compared to the noble horses European cavalrymen and especially nobles rode, the Swedish horses were much more able to survive periods of illness and low to no fodder than their European counterparts, which resulted in Swedish cavalry often having a lower percentage of dehorsed cavalrymen at any given time in a campaign. Lighter horses might have less weight in the charge, but they often had more stamina and lasted longer on the battlefield before they had to rest.