The Grand Harbour's boom defence system was composed of a large winch that lifted (stretched) or lowered (slackened) a metal net suspended underwater in order to secure the harbour from any surface or underwater attack. This part of the boom defence is located at the foot of St Lazarus' Bastion, Valletta and consists of a jetty and buildings were the winch was housed. The system was assembled in 1909 during the construction works on the breakwater but was not put into use until 1935 at the outbreak of the Abyssinian Crisis.

The boom defence jetty is built of hard stone and reinforced concrete and equipped with metal rollers to accommodate the steel cable from which the underwater net was suspended.

The winch room is built of globigerina stone while the rear part is excavated within the rock scarp at the foot of St Lazarus Bastion.

The complex includes crew billets, as well as a turret built in stone with metal shutters which served as a look-out post, a machinegun emplacement or placement for a defensive light.

Mepa scheduled the Boom Defence as a Grade 1 national monument owing to its rarity as per Government Notice No. 276/08 in the Government Gazette dated March 28.