Staff discovered the crumbled bailey wall, which surrounds the Kent castle's medieval walls, when they arrived for work on Monday. No injuries were reported and the area has been taped off.
Tourism manager Simon Curtis said at the scene: "Our first responsibility is for health and safety.
"A scaffolding company is on its way, and hopefully that will minimise the risk of further damage."
Scaffolding and corrugated panelling has been put up around the collapsed wall.
Construction advisers from English Heritage and an engineer were due to inspect the scene today.
"This is not affecting the castle, which remains open to visitors," Curtis said.
The collapsed section is not the medieval wall, but instead a 19th century retaining wall used as an abutment for the medieval section.
In 1215, garrisoned by rebel barons, the castle endured an epic siege by King John. Having first undermined the outer wall, John used the fat of 40 pigs to fire a mine under the keep, bringing its southern corner crashing down. Even then the defenders held out within the building, until they were eventually starved out after a resistance of nearly two months.
Rebuilt under Henry III and Edward I, the castle remained a viable fortress in the 15th century, but a century later it was decaying. It has been restored and is now managed by Medway Council.
In 2006 fears that parts of the castle could collapse due to structural deterioration led to its closure to the public and promoted Medway Council and English Heritage to invest hundreds of thousands of pounds into a restoration efforts.
