Castles
In the past, the territory that is now Germany composed of a huge amount of little kingdoms and duchies and principalities. Obviously every king, duke or prince needed at least one castle, so they have amassed over time.
There used to be around 25.000 castles in Germany, many of which are now ruins. But even if only one tenth of them are still there, that still leaves 2500 castles for you to go and visit.