Napoleon conquered most of Europe with an array of insightful tactics. He used military strength, political maneuvering, forced alliances, annexation and idealism to bring large swathes of Europe under his control.
Napoleon, first and foremost, was a shrewd military tactician and a fearless leader. In addition to clever battlefield tactics, he was prepared to take significant risks and willing to suffer huge losses. This made him fearsome and dangerous to his opponents, and it resulted in some crushing defeats for his enemies, which granted him control over cities, land and thrones. He than annexed these into France, quickly expanding his European empire.
Once he had control over a country, he often used his power to depose the monarchs and install close personal friends or family members on those thrones. A clear example was his choice to depose the Spanish monarch and install his brother as the king, instead.
Often the threat of invasion was enough to terrify a country with weaker military presence into submission. Napoleon used his political influence and military might to force alliances.
He controlled an incredibly successful empire and quelled many objections to his rule by balancing enlightened ideas, military strength, the growth of industry and social order. Additionally, he enforced the Napoleonic code, which made all citizens, regardless of social status, equal under the law.