Is it some kind of a fortress in the background?

The "house" is the "Porte de France".
In the Middle Ages, such a gate had a double function: control over who enters the city (and possibly collect money) and also have a defensive function.
In the immediate vicinity, there were also habitations for soldiers who could keep an eye on the surroundings from higher up.
In many European cities, such gates can still be found today, which have lost their function, but have been preserved for historical reasons.

https://fr.wikipedia.org/wiki/Porte_de_France_(Grenoble)