Corvus Demolitor #1 ("Iron Hand -- demolition raven" )

The ancient sources mention defenders using grapnels  or iron hands for ensnaring men

 and siege machinery.  Diades (a pupil of Phillip's engineer Polyidus) is credited with
the invention of one of these, the corvus demolitor or "demolition raven."
The image in the reference was imaginatively reconstructed by de Folard (authors
collection).  This device was used by attackers to pull stones off a curtain wall.   The
height of a man is about one/fifth of the length of the gripping pole (red head).  As
shown in the book's picture, it seemed to work best when there was no-one on the
defensive wall.  The movement needed to swing the "iron hand" into a solid gripping
situation and then to pull the stone off the wall seems to be more than the defensive
mantlets can screen!  Vetruvius describes the above machine built for
Alexander the Great.

Reference: Duncan B. Campbell, Brian Delf (illus.), "Greek and Roman Siege
Machinery 399 BC -- AD 363," New Vanguard-78, (Osprey Pub., 2003), Page 16
 and 34.  (ISBN 1-84176-605-4)