I could consult this booklet now:
Juliers-France. Jülich in Frankreich 1794-1814 (= Jülicher Forschungen 3), Jülich 1994
It cites (in German translation only) documents from the SHD (former SHAT, both in: État-major de l'armée de terre, article 15, section 3, sièges des places étrangères, carton 18):
A "rapport du capitaine de génie de Bellonet". According to Bellonet, the first commander, "an overaged man [in German: Greis, in French probably; viellard] of 80 years, was replaced with M. de Saint-Louis, hitherto chef of a batallion of artillery [maybe, the French original simply says: chef de bataillon d'artillerie]. Shortly afterwards, a colonel was sent as commander, who later was replaced with general Buque [sic] ..."
And the report about the defense by Buquet himsef, who, having received on 6 January 1814 in Nancy the order to command Juliers, arrived there on 19 January 1814.