Emperor Franz Stronghold, Military fortress in Koblenz, Germany.

Emperor Franz Stronghold is a ruined Prussian fortress on the Petersberg in Koblenz-Lützel. Of the semicircular redoubt, only the two lateral ends and the Kehlturm at the foot of the hill survive; the main gate with its adjoining poterne, the mortar battery in the work yard and the outer wall toward the Bubenheimer Flesche also remain. The walls are grey and tan stone with peeling plaster, and three large arched openings face a cobbled courtyard.

Earthworks on the Petersberg began on 16 September 1816; the foundation stone was laid on 14 April 1817, and the fort was built from 1816 to 1822 to plans by the Prussian engineer officer Claudius Franz Le Bauld de Nans. On 24 November 1818 King Friedrich Wilhelm III named it after Emperor Franz I of Austria, Prussia's ally in the Holy Alliance. It was the main work of the Feste Kaiser Franz system within the Prussian fortress of Koblenz, and in 1870/71 a prisoner-of-war camp for French soldiers stood on the Petersberg.

The fortress is named after Austrian Emperor Franz I, Prussia's ally in the Holy Alliance after Napoleon's defeat. King Friedrich Wilhelm III gave the name during a visit on 24 November 1818. The site is now looked after by the registered association Feste Kaiser Franz e.V., which works for its preservation and opens the grounds to visitors.

The site lies on the Petersberg in Koblenz-Lützel, above the city. Surviving parts are the two ends of the semicircular reduit, the Kehlturm at the foot of the hill, the main gate with its poterne, the mortar battery in the work yard and the outer retaining wall. Ground is uneven and partly overgrown; the arches of the lower level are cordoned off with striped tape.

The fort was named on 24 November 1818, when King Frederick William III visited and dedicated the Petersberg works to Austrian Emperor Francis I, Prussia's ally in the Holy Alliance. The name recalls the coalition that defeated Napoleon, not a local ruler.