The doorknob is a large iron ball.
Several spring-loaded blades emerge from the ball when it is turned.
A DC 12 Wisdom (Perception) check detects several small slits in the ball.
When the doorknob is turned, the blades spring out through the slits. This is a +4 melee attack for 4d4 slashing and piercing damage. The blades immediately retract.
One of the rungs is spring-loaded and snaps into the one above it when pressure is applied.
A DC 12 Wisdom (Perception) check detects a small groove on each side of the ladder above a rung.
When pressure is applied to the rung, it depresses slightly then snaps up into the rung above it, but does not lock in place. The target must make a DC 12 Dexterity saving throw or fall, taking 2d6 bludgeoning damage.
The rung can be bypassed with a successful DC 8 Strength (Athletics) check. Failure results in falling.
The rung can also be secured by placing an object in the depression. This requires a DC 14 Intelligence (Investigation) check.
The springs can also be disconnected, which requires a DC 14 Dexterity check with thieves’ tools.