Where is the kingslayer's hideout?

I'm supposed to go to the kingslayer's hideout now, as part of the Assasins of Kings quest, but I just can't find it. The quest marker points to a cave system near the army camp, but the hideout is nowhere to be seen there. The hideout itself has a very recognizable door (as you saw in the vision), so it should be easy to identify once you found it.

Am I not looking thoroughly enough or is the quest marker in the wrong place?

Thinking 1440 made this nice screenshot:

or go directly to the video which shows how to get there within 1 min, I wasted an hour on this till I saw the video

you can check that same blog as it is a good resource for most places you'll get stuck, it's helped me a lot

Turns out the quest marker is just wrong. The hideout is deep in the south, see Geralt's position in the following map for the true location of the Kingslayer's hideout.