Thanks everyone.
Indeed, the sky was bright. This was shot uncomfortably close to Paris, so don't expect suitable darkness for astronomy, unfortunately. the moon was high and bright as well (4 days before full moon), further reducing contrast.
I have actually already darkened in PS and have reached a point where I am loosing stars if I push further. There is probably a better way to do it. I tried with curves, black point selection, contrast slider...
I will try again, maybe in the absence of the moon. But then of course I will lose the nice light on the castle and the forest.
Anyway I will go back there as I believe I found a better vantage point (but was already 2 hours into this series and had to get some sleep before another long day at work!)
