Background: Archduke Charles of Austria, Duke of Teschen (German: Erzherzog Carl Ludwig Johann Joseph Laurentius von Osterreich, Herzog von Teschen; 5 September 1771 - 30 April 1847) was an Austrian field-marshal, the third son of Emperor Leopold II and his wife, Maria Luisa of Spain. He was also the younger brother of Francis II, Holy Roman Emperor. Despite being epileptic, Charles achieved respect both as a commander and as a reformer of the Austrian army.
Context: The caution which the archduke preached so earnestly in his strategic works, he displayed in practice only when the situation seemed to demand it, though his education certainly prejudiced him in favor of the defensive at all costs. He was at the same time capable of forming and executing the most daring offensive strategy, and his tactical skill in the handling of troops, whether in wide turning movements, as at Wurzburg and Zurich, or in masses, as at Aspern and Wagram, was certainly equal to that of any leader of his time, with only a few exceptions.  According to the Encyclopaedia Britannica Eleventh Edition, his campaign of 1796 is considered almost faultless. That he sustained defeat in 1809 was due in part to the great numerical superiority of the French and their allies, and in part to the condition of his newly reorganized troops. His six weeks' inaction after the victory of Aspern is, however, open to unfavorable criticism. As a military writer, his position in the evolution of the art of war is very important, and his doctrines had naturally the greatest weight. Nevertheless, they cannot but be considered antiquated even in 1806. Caution and the importance of strategic points are the chief features of his system. The rigidity of his geographical strategy may be gathered from the prescription that "this principle is never to be departed from."  Again and again he repeated the advice that nothing should be hazarded unless one's army is completely secure, a rule which he himself neglected with such brilliant results in 1796. Strategic points, he says, not the defeat of the enemy's army, decide the fate of one's own country, and must constantly remain the general's main concern, a maxim which was never more remarkably disproved than in the war of 1809. The editor of the archduke's work is able to make but a feeble defense against Clausewitz's reproach that Charles attached more value to ground than to the annihilation of the foe. In his tactical writings the same spirit is conspicuous. His reserve in battle is designed to "cover a retreat."  The baneful influence of these antiquated principles was clearly shown in the maintenance of Koniggratz-Josefstadt in 1866 as a strategic point, which was preferred to the defeat of the separated Prussian armies, and in the strange plans produced in Vienna for the campaign of 1859, and in the almost unintelligible Battle of Montebello in the same year. The theory and the practice of Archduke Charles form one of the most curious contrasts in military history. In the one he is unreal, in the other he displayed, along with the greatest skill, a vivid activity which made him for long the most formidable opponent of Napoleon.  He was the 831st Knight of the Order of the Golden Fleece in Austria.
Question: what was his biggest achievement?

Answer:
his campaign of 1796 is considered almost faultless.