How to get to the ruins of the lonely rock fortress? (The Witcher 3: Wild Hunt)

you walk along the bridge to the cliff, you fall down, climb the stones, profit, he just did it himself and yes, there is a wyvern in front of him.
Well, or there is a place of power below, under it there is a cave in the water - you float inside, there is a passage, and you will get into the tower

down there under the water there is a passage inside ...

http://steamcommunity.com/id/Amadara/screenshots

screen

There is a tunnel under the place of power. The entrance is underwater.