Tokyo Imperial Palace
At its height, Edo Castle was humongous. Nothing remains today except the renovated ramparts and gates, and the base of the keep. The castle was never attacked, and it never fell, although it was destroyed by accidental fire more than once. Every time I walk around it I try to imagine how it could have been attacked. The moats are deep, wide and steep. The walls are huge. Atop the walls, more walls are hidden behind the trees. The gates are killing zones. Every space is pre-sighted for archers' arrows. In Japan's feudal period, there were several rings of moats and walls, mostly gone now, hampering, guarding, and regulating the approach. The Tokyo Imperial Palace today is only the innermost core of the original, sprawling castle. It was a mighty fort, and an engineering marvel. The castle itself was its own city.
Today, I imagine how terrorists might try to sneak in to assassinate or kidnap the Emperor, or something. The short story is that it's impossible. First, intruders would have to contend with the moat that still exists. But the moat today is seeded with American snapping turtles and Amazonian piranha fish. But avoiding those perils, and successfully penetrating the walls, attackers would then face the Imperial Household Agency guards, who are pretty serious. Every guard post has direct line-of-sight to the next post, and the grounds are well patrolled inside and out. Finally, there is all the electronic surveillance and warning systems in place.