During the French occupation of Malta in 1798, he was chosen as a representative of the municipality of Żurrieq, Safi, Kirkop and Gudja.
[1] After the French surrendered and Malta became a British protectorate, Castagna was appointed as the luogotenente of Senglea and Cospicua.
[2] In November 1801, he was part of a delegation of six representatives[a] who travelled to London to present the needs of the Maltese people to the British government.
[3] After the death of Emmanuele Vitale in 1802, Castagna was appointed Governor of Gozo,[2] and he held this position until it was abolished in 1814.
[1] He died on 26 January 1830 after a long illness, and he was buried inside the Għaxaq parish church.