This place is shaped more or less like a bone. It's two houses, one an old turn-of-the-century farmhouse, the other a modern pre-fab, connected by a hallway. Each wing has its own basement.
The "old" basement is small, roughly 10 foot by 10 foot, with a small casement window at ground level. It is accessed from a wooden trapdoor on the porch; the passageway down is lined with the same concrete blocks the basement is made of. The basement door is old, fragile, and doesn't close properly from the inside. The ceiling is the floor of the kitchen.
The new basement is quite large, filling the entire space underneath the addition, about 30 feet by 50 feet. The walls are made of concrete blocks, similar to the old basement. It has a large sliding window in one corner, which is above ground, and two casement windows in below ground wells. The basement is accessed by a wooden stairway from inside the house. The basement door is a standard hollow interior door, but it closes from either side. The ceiling is the floor of the prefab.
Then there's the well box, which is a concrete bunker about 5 feet by 5 feet, extending about 10 feet underground (the actual well extends down another 20 or so feet, but is sealed). It's a good 10 or 15 feet from the house. It has no windows, just two small access holes about the size of a silver dollar. It is accessed from a steel manhole and a wrought iron ladder; the manhole cover has to be rotated to a specific position to lift it. It has no lighting, though currently there's a shop light hanging from an extension cord. The ceiling is solid concrete.
Now the dilemma is, which would be the better shelter? The well box is obviously strongest, but it requires running outside and futzing with the tricky manhole cover; it's also highly claustrophobic down there. The old basement is fairly secure, but the trapdoor is easily blocked and the basement door cannot be secured from the inside. The new basement is easy to get into and offers lots of light and plenty of room to move around, but it's also the least structurally sound.