Could a medieval castle hold off a modern army for even a single day?

A medieval castle would be unable to hold off a modern army for even a single day.

While castles were designed to withstand arrows, catapults, and battering rams, they are largely indefensible against modern military technology for a few key reasons:

• Precision weaponry: Modern artillery and guided missiles can target specific structural weaknesses from miles away. A castle’s thick stone walls, designed to absorb the kinetic energy of slow-moving projectiles, would quickly crumble under the force of modern high-explosive or armor-piercing rounds.
• Air superiority: Modern air forces would bypass the walls entirely. Precision-guided munitions dropped by aircraft or launched by drones could destroy the castle’s internal structures, gatehouses, and defenders without ever needing to breach the perimeter in a traditional sense.
• Range and surveillance: Medieval defenses relied on line-of-sight and relatively short-range weaponry. A modern army would engage the castle from well beyond the range of any defensive equipment the castle might possess, utilizing thermal imaging and satellite reconnaissance to neutralize threats before they could be seen.

In short, a medieval castle is a static defensive structure built for a different era of warfare. Against the speed, range, and destructive power of a modern military, it would be effectively neutralized within minutes, if not seconds.