Ah-hah, the old 'prisoner in the tower' problem, probably originating from a lecture on the half-hitch and later accepted into legend. Apart from its lethality, it does have certain other drawbacks, eg, persuading the guards to bring one 100' of stout cordage with one's supper tray, finding something to anchor it to after cutting the bars away, swimming a shark-infested moat with a coil of soggy rope around one's neck, etc.
My favourite version of this Patrick O'Brian's in which the heroes persuade a washerwoman to provide them with all sorts of fancy rope and tackle and then complain about the quality of the sister blocks. Brilliant irony.