The 10-mile ring is being constructed with panels of steel-mesh fencing. The fence is just of of many security measures that are being put in place for the summit occurring on September 4 and 5. Some residents of Cardiff and Newport are agitated by the measures while others understand the need for tighter security.
"It looks a bit like a zoo," said one resident while another pointed out that the castle already had a moat.
"They do have to secure the castle because of what's going on in the world at the moment and because [U.S. President Barack] Obama is coming," said a woman from Cardiff.
In addition to perimeter security, each world leader will require the individual security as well. There will be 150 heads of state and ministers, 10,000 support staff and 2,000 journalists in attendance.
The government is not disclosing the cost of security, but the BBC reports it is an estimated 50 million pounds ($83 million).
Discussions about Ukraine, Iraq, Syria and the threat of the Islamic State are reported to be on the docket.
"There will always be an element -- there is often an element -- particularly with large, world-scale events that come to try and disrupt and cause harm and disruption to communities as well as the event itself. We will be well prepared for those individuals. They'll be dealt with fairly but firmly," Superintendent of the Gwent Police Mark Warrender told the BBC.
There is a planned "Day of Disruption" during the event, but protesters say their actions will be peaceful.