Yikes to the floor of nails. Yeah you're right, I don't know if the iron gate is needed, that area seems like it would be impossible to traverse. Your feet would get stuck in the nails and you could topple over if you lost your balance - look out hands!
Seeing the method of entry was pretty cool.
If the bars were not there, other than the ground floor.. it would be off-limits. Mills are hardly exciting to begin with.