Calais
Calais is the port town across from Dover on the English channel to which travellers arrive when going from England to France. It is where Chauvelin chases Percy after they both flee Dover in search of aristocratic fugitives.
citoyen
Citoyen is the French word for citizen. Various characters are referred to as 'citoyen' by Chauvelin and his other French henchmen.
Comtesse
Comtesse is the French word for Countess. The Comtesse de Tournay, then, is married to a Count, namely the Vicomte de Tournay
Dover
Dover is a small town on the southernmost tip of England. One can cross from Dover to Calais through the narrow Strait of Dover.
Madame de la Guillotine
Madame de la Guillotine is the nickname that the French people gave to the guillotine, an instrument designed for executing people with a huge blade that drops from a pulley to decapitate its victims.
noblesse
Noblesse is the French word for nobility.
Place de la Greve
Place de la Greve is the square in front of the main government building in Paris - where the public guillotine executions were held during the 'Reign of Terror.'