"Ship's Prow Tortoise" (Roman Siege Machine) #1

This machine was used on slopes leading up to the walls of the city or
fortress under siege.  It's purpose was to protect those men working on
the slope area digging trenches and erecting various siege field
fortifications and other siege machines.  The extreme prow of the
machine was sheathed in iron in order to shunt aside any flammable
engines used by the defenders such as barrels of oil and round packages
of straw rolled down the hill in an effort to discourage the attackers.
This picture view of the tortoise shows it upside down.  The
Scene (Plate E) shows the 'Ship's Prow Tortoise' with associated
earthworks.  Appolodorus opens his 'Poliorketika' with the above scene
of an assault on a hill town, in which he emphasises that the
besiegers must guard against heavy objects being rolled downhill.
He lists in particular, tree trunks, round boulders, heavily laden wagons,
and barrels filled with gravel or earth.  First, he recommends the digging
of a 5 ft. (1.5m) deep ditches, running obliquely downhill; the spoil
from the ditches forms a rampart, to break the momentum of the objects,
and the ditches are intended to channel them away from the main
besieging force, waiting further downhill.  Next, he explains that
the men digging the ditches should be protected by a slanting palisade
line, boarded over and interwoven with branches to form an 'outwork'
(proteichisma).
Finally, the key element in the scheme is the 'tortoise shaped like a
ship's prow'.  Appolodorus' brief description suggests a vertical-sided,
open-topped shelter with a triangular ground plan, arranged so
that the apex, facing uphill, would deflect rolling objects to either
side.  Here it is assumed that the walls would be sufficiently high to
conceal the soldiers crowded inside.

This scene (Plate E) is based on the siege of a hilltop stronghold that
 appears on Trajan's Column and is perhaps intended to represent
the Dacian capitol, Sarmizegethusa.  It has been assumed that the
polygonal masonry of the murus  Dacicus was surrmounted by a
timber breastwork, sections of which could be easily removed to
allow heavy objects to be rolled down against Apollodorus'
ship's prow tortoises.

Reference: Duncan B. Cambell, Brian Delf (illus.), "Greek and
Roman Siege Machinery, 399 BC -- AD 393," New Vanguard-
78, (Osprey Pub., 2003), pages 45-46. (ISBN 1-84176-605-4)