Sambuca

The most famous examples of the sambuca (or sambyke) were mounted 

aboard ships, where they resembled giant laddered drawbridges for thetransfer of marines onto seawalls  of coastal towns.  However, the sambuca that Biton, a writer-engineer writing at sometime between 231 and 133 BC attributes to Damios, an otherwise unknown engineer from Kolophon in present-day Turkey, is quite different.  First, it was designed for use on land, and second, it utilized an innovative vertical screw to alter the elevation of the ladder.  Biton says that the sambuca itself , a 60 ft (18 m) ladder with an assault platform at one end, and a counterweight at the other, sat on a 'trestle' (killibas); the trestle was fixed to a 27 ft. (8m) undercarriage.  Biton's ladder clearly has sidewalls, 'so that men climbing up will make the ascent confidently', and a widened jumping off area at the top.  Biton's description of the Dammios sambuca is extremely concise.  For the undercarriage he simply gives the beam dimensions of 3 ft. x 2ft. x27 ft. (0.9m x 0.6m x 8m) and notes that the wheels were 3 ft. (0.9m) high; here a rectangular under carriage with six wheels is assumed.  The main component , a 60 ft. (18m) ladder was hinged to the rear of the tresle, which supported a centrally located vertical screw; the screw's function was to raise and lower he main ladder.  It is reasonable to suppose that as much of the machine as possible was boarded in, in order to protect the crew.  Biton specifies that the trestle was 14 ft. (4.2m) high, whereas the screw was 15 ft. (4.5m) long:, consequently, when fully turned , it would project 1 ft. (0.3m) above the trestle.  Assuming a forward projection of around 12m for the ladder, with the screw fully turned, the assault platform rose almost 9m above ground level.

Maintaining the machine's stability would have been a delicate task.  Withe the main ladder in it's horizontal position, the vertical ladder at the front was perhaps to support the machine while the assault team took up position on the forward platform.  Their presence there would have severely unbalanced the machine, so the counterweight at the rear must have been intended to restore its equilibrium.  Only then would the main ladder have been elevated  and the machine rolled forward.

Reference: Duncan B. Campbell, Brian Delf (illus.), "Greek and Roman Siege Machinery, 399  BC --- AD 363," New Vanguard-78, (Osprey Pub., 2003), Pges 24 and 44. (ISBN 1-84176-605-4)