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Castellani designed a number of prominent buildings in Guyana, including Brickdam Cathedral; Castellani House, once the Guyana Prime Minister's residence; the Brickdam Police Station; and the New Amsterdam Public Hospital.
He made alterations and additions to the Church of the Sacred Heart on Main Street (1872-1882), which opened in 1861, for the colony's Madeiran labourers.
In 1875, he completed the installation of a sunken panelled ceiling of the Parliamentary chamber in the eastern wing of the Parliament Building.
Cesar Castellani died in Georgetown, Guyana on 2 August 1905.