How do I get through the locked door in Freya's Garden?

I'm in Freya's Garden on the "Nameless" quest. I tracked Craven until I stood in fron of a locked door at the point marked on this screenshot:

I can't open the door, and haven't found a way around it so far. The quest description is not really helpful, it just states "Explore the further parts of the herbarium".

How do I get through that closed door?

Solution 1:

Look for the stone ladder:

It is where Geralt looks at. It will not be shown from the Witcher's senses.

Solution 2:

If you go all the way to the right of the garden you will find a ladder. Head up those and you should get a cutscene about the floodgates. I don't really know what they do yet (still doing the mission) but if you keep going you should come across the werewolf's cave where you can fight and talk to him.