Police say youths are "putting their lives at risk" by illegally climbing on Newark Castle.
The castle is hundreds of years old and so can be easily damaged, which in turn costs the council and taxpayer a huge amount to fix.
Nottinghamshire Police is now appealing for information after reports of young people climbing along the castle's wall, which is a "serious safety risk" for those involved.
Youths have also been spotted climbing on the top of surrounding buildings and shops in the town centre.
A spokesperson for Newark Police said: "The walls of the castle are hundreds of years old and are of great historic value.
"Anyone that climbs the Castle walls is putting their life at risk due to the height and age of the structure.
"These are historic buildings that are there for the enjoyment of residents and visitors to Newark.
"Damage is easily caused to the buildings and this could have a huge financial effect for local businesses and for the district council."
Anyone with any information should call 101, or 999 if they spot anyone doing it at the time.
