Governors of Jersey moved their official residence from Mont Orgueil to Elizabeth Castle, which was first used in a military context during the English Civil War in the 17th century. and was built to defend jersey against French invasions and has witnessed many significant events over the centuries. some of the wars that it was used in consist of The English Civil war, the Napoleonic Wars and world war 2.

it took around 70 years to complete the construction of the castle. It was also built in stages, with different sections being added over time. The construction began in the 16th century and continued in stages until the 17th century.

Built on a rocky islet in St. Aubin’s Bay, Elizabeth Castle has defended Jersey for more than 400 years. You can take the amphibious castle ferry or walk out along the causeway at low tide. Elizabeth Castle is the perfect place to spend a day exploring Jersey’s history with the Jersey Militia or castle gunner on parade.

Below is an old map of the castle

The main weapons that they used for defence consist of cannons, muskets and other firearms that where commonly used during that period.

below is some more information that i found on this website

The islet

Elizabeth Castle, as its name suggests, dates from the days of the great Tudor Queen. But its site had long previously been associated with events of local importance, the earliest of which might well be termed legendary rather than historical.

Here, about the year 540 AD, came Helier, the Christian anchorite after whom our town is named. He chose as his habitation a lonely spume-sprayed rock south east of the islet, where his reputed bed, a rough niche in the rock, may still be seen. In the same place, some 15 years later, he was discovered by a band of marauding sea-rovers and put to the sword, thus earning a place in the Calendar as Jersey’s first Christian martyr and patron saint.

The Hermitage, or oratory, which encloses his cell was erected at a much later date, probably in the 12th century.

St Marculf

St Helier was a pupil of St Marculf, to whom is chiefly attributed the conversion of the islanders to Christianity. As the population of Jersey then numbered, we are told, only 30 families, the task was not quite so formidable as it sounds. But the benefits conferred by these pioneers of Christianity were material as well as spiritual. The monasteries which they founded invariably attracted settlers to the district, agriculture and the rudiments of education were taught, and an impetus given to civilisation generally. Shortly after the death of St Helier, St Marculf established a monastic settlement upon the islet, and the agglomeration of peasants’ huts which, as a consequence, sprang up in the near vicinity, formed the foundations of the future town of St Helier. After the death of Marcuif in 558, his work was carried on by St Magloire or Mannelier. In 577 Pretextat, Archbishop of Rouen, as the result of a quarrel with the Frankish King, fled to Jersey and took refuge in the Monastery of the islet. He was the first of the host of political exiles who, throughout history, have found shelter and security on our shores. Jersey’s inclusion in the Empire of Charlemagne is disclosed by the records of the Abbey of Fontenelle, which state that the Emperor sent Abbot Gerwold on a diplomatic mission to Augia, as it was then termed, in 790.Towards the end of the ninth century, Norse raiders again descended on the Islet and St Marculf’s Monastery was laid in ruins.