Just guessing here. But a shelter need to have mechanical ventilation, and a way to escape if the main door is blocked. So modern shelters usually have a plate at a wall, below ground, and when removed, lets dirt from the outside fall in and provides a way to leave the shelter. Such things may not have existed around 1900.

> plate at a wall, below ground, and when removed, lets dirt from the outside fall in

..collapsed parking garage with four stories above you of reinforced concrete rubble and knotted, melted, corkscrewed rebar? Respect the Swiss, they have a history of collectively trying harder than anyone else.

That's a really good idea I might never have thought of. But I hope they include a shovel or pickaxe or something in the shelter. I'd be worried about the soil in front of the entrance being hardened or full of tree roots.

Yes, according to information I found, shovels are part of the kit.

Newer ones in Finland tend to have these smaller escape hatches + tunnels that lead away from the building. The main blast door being inside the building

The article talks about the standards that modern shelters are built to (reinforced concrete, air filtration, water, etc).