Moon labyrinth (passageway detail)

The patience for this work is remarkable, the idea is beyond me but it appeals as an image to be explored, can you advise what pen you used please.

Thanks Derek appreciate your feedback as always ... I have recently had a new pen purchased by my grandad which I will use for future designs for this one I used a z grip smooth medium zebra pen which is like a biro. my new pen will be a Japanese pen which I will put in the future description. I thank you for your continued viewing of my work :)