Porgi the Turtle

Case in point: Tsun Zu's rival defended a city with 10 men against Tsun's army of hundreds by disarming his own soldiers, dressing them in plain clothes, INVITING Tsun's army to come in, and it only worked because Tsun knew the guy was an ambush master and thought "if we attack the city he's inviting us into, we will die." and left without even trying ON THE BASIS OF HIS RIVAL'S REPUTATION AND NOTHING MORE

Another example: Tsun Zu, on being told his soliders were out of arrows during a battle against a city across a river from them, had his men craft scarecrows, put them on a boat, send it out on a line, leave it there for half an hour, then pull it back in and used the arrows the enemy had fired at the boat to restock their own ammunition. It only worked because it was foggy and the enemy couldn't tell the difference between the scarecrows and actual soldiers.

so you can start with a thing like "nobody would expect an attack from underneath the castle!" and then design your castle with :

feature that allows this (catacombs from before the ancient cathedral was renovated into a full blown castle)

reason nobody would expect it (the renovators sealed off the catacombs, current occupants don't know the catacombs exist)

genius reason Our Great Hero thinks to make use of this (his common sense but deeply insightful assessment causes him to question where the rain water drains from the multiple terraced courtyards and grand balconies (the renovators did leave a drainage system that exits via the catacombs, which works so well that the current occupants never had reason to wonder about water drainage)

one or two additional things that help make it genius (Our Hero knows the castle used to belong to the original cathedral people, some of whom still live nearby and are bitter about losing the castle to the current occupants)

optional: additional improvised stroke of genius during the event (Our Hero finds current occupants legendary un-beatable foe [previous occupant's great grandfather] interred in catacombs and leads the invasion of the castle dressed as said legendary foe in his very recognizable armor that has clearly been sitting in a crypt for a hundred years.)