Possible Clues:

• Access to a treehouse, maybe
• Obstacle course component, often
• Ratline
• Shaky steps up to the gallows?
• Prole slips on something snaky - an etrier
• What's got person up into tree? One's stranded
• Pal rode off, embarrassed after climbing this?
• Run on moor - not a stiff route to the top
• It might be used for an emergency exit
• Boarding aid, at times