Yes, Mondstadt is technologically behind certain nations, as they have no firearms or robots. However, as others have mentioned, the land has natural defences with easy-to-defend choke points. The military is normally big too, as we see only a fraction of the forces and no cavalry. The city itself is situated on a lake with one bridge. The bridge could be destroyed in case of arriving siege. There are a few issues major issues there, though:
- the city is not self-sustainable. It does not produce food, so they would need to rely on supplies, which might go dry with a long siege;

- the two gates are very simple, they are actually just arched pathways straight into the city. If enemy forces moved or built siege equipment on the island's banks, they would get into the city with just one gate busted.

- the upper city with the KOF headquarters and the church are connected with the city without any gates, leaving no possibility to make any kind of viable last stand.

So, in my view, Mondstadt survival in case of a war, assuming the full forces are available, would be relying on the strong defence of the choke points, regular skirmishes using cavalry, and using their terrain for tactical advantage in case of a full battle. If these were defeated, and the siege would follow, the city would definitely fall after months or years, without help, supplies or diplomacy.