Takeda Castle Ruins are located in Hyogo Prefecture. They are well-known as the “Machu Picchu of Japan.”
The Takeda ruins have become more famous recently because of their appearance in commercials and movies and the influx of people have caused them to close the mid-way parking lot to passenger cars. Buses and taxis can go to the parking lot.
If you go by car, you have to stop and park at Yamajiro no Sato parking area (it is a 20 minute walk from there to the mid-way parking lot, and then another 20 minute walk to the top). You can also walk to the mountain from Takeda Station, it is about 40 minutes on a walking trail.
It is about a 2.5 hr drive away from Osaka.
It is free for now, but they are going to start charging an entry fee soon.
Most people will want to go to the neighboring mountain, Ritsuunkyo, where you can experience the world-famous view of Takeda ruins floating in the clouds. The viewpoint is 20 minutes away from Takeda Station by car and an hour walk by foot.
Here are some pictures of the ruins (unfortunately it was a day when the cloud cover was not as thick as the picture above).
It will probably take you 1.5 hours to 2 hours to walk around the ruins. Probably the best times to visit would be in the spring and the fall, just because in the summer it is going to be hot and there is no shade to hide in, and the winter brings a lot of snow.