Well, you didn't specify the time period. My best guess would be that you meant Germany during 20th century, under Hitler's regime.
Germany itself is not the best place to go to war from, especially when it comes to Eastern and Western front; however, its Southern front is well defended by mountains.
Since its southern front is naturally defended, it added up to a huge list of reasons why Austria agreed for the Anschluss.
Looking at its geographical map, we see that beaches are quite elevated as well, making it quite hard for allies to properly naval invade it.
There are plenty of rivers in Germany, and all of them are like mini-maginot lines when used properly. This certainly stopped Soviet Troops advancing into it, but by that time, it was already too late.
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