"The Gun" by Philip K. Dick is a science fiction short story first published in 1952. Space explorers investigating what seems to be a deserted planet are mysteriously shot down and forced to crash land. Their search for answers leads them to ancient ruins and a deadly automated weapon programmed to destroy anything that flies overhead. The gun protects something precious beneath it—remnants of a lost civilization. But destroying their mechanical enemy may not be enough to escape this world alive.
Wikipedia page about this book: en.wikipedia.org/wiki/The_Gun_(short_story))