"Magnae et invictae Britanniae, Melitensium amor et Europa vox has insulas confirmant" (the inscription on the Main Guard portico misspells the final word as 'confirmat')
To Great and Unconquered Britain, the Love of the Maltese and the Voice of Europe Confirms these Islands.
The "Voice of Europe" is a euphemism for the Treaty of Paris 1814, a Constitutional Statute signed May 30, 1814 to end the Napoleonic Wars, and in particular reference to Article 7:
"The Island of Malta and its Dependencies shall belong in full right and Sovereignty to His Britannic Majesty."
Thus, 1814 is when Malta became part of the British Empire as a Crown Colony.