The summary is:
The speaker is visiting a haunted castle hotel in Germany with his friend Calvin. They record their experiences with a device that detects ghost activities. They encounter some strange noises and events in the hotel and the nearby tunnel. Calvin disappears in the tunnel and the speaker tries to find him, but fails. He decides to go back to the hotel and hopes that Calvin is there.
Here are the key facts extracted from the text:
1. The text is a transcript of a video where the speaker visits a haunted hotel in Germany.
2. The hotel is called Hotel Adlon and it is located in a castle that belonged to two brothers who fought over the inheritance and died.
3. The speaker has a device that shows green or red depending on the ghost activity level.
4. The speaker experiences rain, thunder, noises and other strange phenomena in and around the hotel.
5. The speaker loses his friend Calvin in a tunnel and hears screams from the other side.