Born into a Dutch-Huguenot family; he notably held the fortress of Graudenz against Napoleon's troops throughout 1807, long past the Peace of Tilsit, until the siege was finally lifted after 11 months.

He served with the Regiment d’Aylva in the War of the Austrian Succession, participating in the defense of Bergen op Zoom.

Gaining the attention of King Frederick the Great, after the Siege of Schweidnitz (1758) he was given the rank of Major and command of a free battalion.

Then, after briefly leaving the field, they reoccupied the city and, by now commanded by General Johann Georg von Schäffer-Bernstein, started besieging the fortress again.

[8] In March Napoleon, impending to besiege Danzig (situated down on the end of the Vistula), sent General Anne Jean Marie René Savary to demand surrender again.

[Note 2][2][5][9] In the following months the siege was intensified, the French got more reinforcements and after Danzig fell command was briefly given to Claude Victor-Perrin, Duc de Belluno before Rouyer took over again.

[11] Meanwhile, the French, against the statutes of the signed treaty, upheld their blockade of the fortress and continued to occupy the area; their troops now consisted mainly of Saxons under General Georg Friedrich August von Polenz.

After the borders between Prussia and the new Duchy of Warsaw were finally set, with Graudenz remaining Prussian territory, on 12 December the blockade was lifted and the last French troops left the city.