The star fort was added by the Default3+ ruleset, and you could find the answer there. But I'll tell you anyways: In Default3+, the city walls don't protect your cities against post-gunpowder units that good, and don't protect them at all against modern units. The Star Fort is basically Level 2 city walls, it protects your city against Gunpowder-era units pretty good, and offers a slight protection against modern units. The City Defense is the Level 3 version, which protects against modern units too.