The engineers of the Death Star get a lot of criticism for designing a moon-sized battleship that can be destroyed with a single shot in one vulnerable spot (assuming that you believe the official story). Many members of the design team have been unable to find professional employment and several have been sued into bankruptcy. This is largely because of a public misunderstanding of the nature of engineering on a project the scale of a Death Star.
Abstruse Goose makes an important point: every complex system, including the male human body, has vulnerabilities. Human testicles are outside of the body for an important reason: to keep sperm cells cool enough to remain viable. The Death Star had an exhaust port for a similar reason: to keep the station cool enough to remain operational within tolerances.
The Death Star was properly designed. It just wasn't properly defended.