Its a fair old treck. I did it back in october 2018 with a very determined but very poorly Mrs Bobley who just wanted to see the Castle just once. She'd walked almost everywhere else so she wasn't going to miss this bit.
We parked the car in a little layby about 850m away from the gate and wandered up to meet. There was a mixed crowd ranging from early retirees to families with kids c. 12y.o.
The walk was very gentle and we stopped a lot for the guide to explain about the wildlife and the how important it was to protect the vultures at the castle as they are endangered species. At this point someone none english would ask whether we would be going to the castle and the guide would say "I'm afraid not...".
We would travel on under tree cover until we got to the valley and the guide gave us all binoculars and told us to look up here at this bird and over there at that thing which was fascinating and then the next none english couple would ask "when would we get the to castle" and the guide would remark again that we weren't going to the castle" and you could see the colour of his face change colour ever so slightly.
Once the castle was in view we would repeat the same loop of question and answers and we'd trot on until we got to the end of the valley and were looking down at Cala Castell. We'd stop for lunch at this point and folks chattered amongst themselves and the guide would talk to folks about birds if they asked nicely.
Then, we set off back home... but there was one last chance for a French couple to ask about the castle.... the guide just... held it together but you could see he was on the verge of going full on Basil Fawlty. It was good entertainment. About 5 hrs? 10 miles of slow walking, but you dont rush it
Anyway, today, things have changed and allegedly (acc to the Majorca Daily Rag) you can get to the castle!! So I might give it another shot... but I'll go up to the gate on my bike so save a bit of a walk!!
https://www.visitaguiadaternelles.com/