But it's not reallly about him, I'm just curious what a regular soldier from a place like Dusseldorf would face, in what I imagine were pretty turbulent times in Europe.
Random facts, which may or may not factor into an answer: He gets married in 1815 in the Dutch city of Groningen, to a Dutch woman. He is 30 at this time, which seems pretty late to get married, but maybe that was normal for military men. Judging from his job descriptions and frequent address changes on the subsequent birth certificates for his children, he's getting by with odd labor jobs and eventually works as a garbage sweep. But sometimes he is still listed as "retired soldier". If he recieved an actual pension it would've been not enough to live from, I reckon.
If his soldiering was done for the government controlling Düsseldorf, then he did have some pretty turbulent times.
At the beginning of the Napoleonic Wars, Düsseldorf was part of the Palatinate, which was held in personal Union by the Duke of Bavaria in 1785. Bavarian and presumably Palatine troops fought against Napoleon in the War of the Second Coalition. After their defeat, Bavaria seceded from the Holy Roman Empire and, in exchange for various territorial cessions, allowed Napoleon to establish the Grand Duchy of Berg, including Düsseldorf. Berg was ruled more or less directly by Napoleon and his proxies until his defeat and exile to Elba, and during that time, troops from Berg and other allied German territories were included in all of Napoleon’s major campaigns and used as garrison troops throughout the Empire.
Without knowing more detail about his life it’s difficult to say anything for sure. But a soldier of your ancestor’s age would have been eligible for military service during almost the whole of the Napoleonic Wars, so he could potentially have seen some of the most famous battlefields of the era.