Back in the '90s I remember seeing East German bunker periscopes offered for sale in the Sovietski catalog. Thought it would have been great mounted on a storm shelter, I wish I'd bought one.
The one I have is a 4-9X magnifier. The objective lens extends out form the body about a foot or so, and seems to be well-built. Images are clear and sharp. The one concern I have is that it looks somewhat delicate, but if it is kept in a fixed shelter and not banged around, it should work well.
Mine is a tad more sophisticated than that. Think of it as a prismatic zoom monocular, because that is basically what it is.
It enables the viewer to see around corners in relative safety. I have seen on TV that the US military uses tripod-mounted binocular periscopes along the DMZ in Korea.
