Paramedics from the Isle of Wight Ambulance Service have been joined by the flying medics, alongside fire crews from the Isle of Wight Fire and Rescue Service, at Baring Road.
Fire appliances from Cowes and Newport are on scene at the grade II listed arched gateway building dating back to 1841.
An individual is understood to have suffered a fall from height. As a result, fire crews are utilising an Aerial Ladder Platform (ALP) to assess the casualty, who appears to be behind a wall.
There are 3 flights of steps that lead through the centre of the historic building to Northwood Park from the top of Castle Hill, near to The Parade.
The mainland air ambulance has landed in the grounds of Northwood Park to the rear of Northwood House.
The incident began at around 22:00 with the air ambulance landing in Cowes at around 22:40.
Baring Road, Castle Hill and Castle Road have all been closed to traffic and pedestrians with a police cordon in place.
UPDATE @ 23:25 – 1 casualty has been packaged into a stretcher and lifted using a pulley and rope system attached to the underside of the fire service’s ALP.
The injured individual has been transferred to firmer ground and is expected to be taken to hospital in due course.
Exact circumstances of the incident are yet to be confirmed.
UPDATE @ 00:00 – The casualty has now been transported to an awaiting ambulance and is being taken to St Mary’s Hospital in Newport by land.
Crews are standing down from the incident and the Dorset and Somerset Air Ambulance is expected to take off and return to base shortly.
UPDATE @ 00:44 – It has been confirmed that the casualty fell around 20ft into a void, known locally as The Pit.
It remains unclear as to why the person was on top of the building in the first place.
Firefighters have used ladders, work restraints, rope rescue equipment and the aerial appliance to effect a successful rescue.
The scene has been cleared and cordons lifted.
The air ambulance has in the past few minutes lifted off from Northwood Park after an extended period on the ground.