Wouldn't he have a lack of troops? Why did the Allies take so long to invade Germany?
Also, how did he manage to control so much land?
Let's first start of by pointing out the obvious. There were two naval invasions during Hitler's rule of Europe. First the allied invasion of Italy starting in 1943. Later the invasion of France starting in 1944. On most scales two failures wouldn't be called a successful defense.
On the western coast of France the Nazis ordered the building the Atlantikwall, a huge defense structure from the north of Norway, to the French Pyrenees. /u/thefourthmaninaboat explains why it couldn't have worked even if it had been fnished.
The invasion of Germany took so long for several reasons. One was that it is hard to go from Italy across the Alps into Germany ^1. So they had to do the D-Day invasion. That took time because they had to build and gather the forces necessary for such an invasion ^2. In the USSR people at the time suspected the allies weren't hurrying to invade because they preferred it if the Red Army did the dying^3 . It was a combination of those factors that mean that Germany wasn't invaded from the West before 1944.
I'm not too well informed about details of the occupation, but it wasn't the same for everyone. In the East the Nazi occupation was harsh and often deadly. In Bloodlands Timothy Snyder describes how people were intentionally starved in those regions. In occupied France and Denmark they still had their own government, which then cooperated with the Nazi commandments. In other European countries the Nazis took over the government entirely and ruled in their own right ^4. In all of the occupied nations, the Nazis used violence and the threat of violence to repress resistance against their occupation.
