Meghan Markle and Prince Harry's royal wedding security will cost $33 million and here's why.

Dai Davies, former Head of Royal Protection and Chief Superintendent of the Metropolitan Police service, said that the venue is difficult to secure. In addition, all types of people may flock the Windsor Castle, including those who don't support Prince Harry and Markle's union.

"Windsor is a much harder area to secure than London, which is so much better prepared. In Windsor, there are shops, buildings, houses, you name it and there is a history of people trying to get into Windsor Castle," Davies told E! News.

"The greatest risk," Davies continued. "comes from those people that we call 'fixated', i.e., mentally ill people, who could think, for whatever reason, that a white prince shouldn't marry a black woman."

"Clearly 99 percent of the population are deliriously happy about that but you could get a small right-wing fascist group who may object to it, so that adds to your problem. I don't know how they are going to secure it [Windsor] to the level that I would want, but clearly, much wiser people are now doing the job and I'm sure they'll come up with a plan."

Dr. David James, a psychiatrist who formerly worked with the Fixated Threat Assessment Centre, supported Davies' concerns. "Royalty tends to attract people who have strange delusions and are mentally ill," Dr. James said.

According to the medical expert, some individuals may think that they are related or are married to the concerned couple. Others may put the blame of their condition to the royal family.

What makes Markle and Prince Harry's royal wedding more challenging is that the couple is having a carriage procession. Many deem this unnecessary as the groom-to-be is already fifth-in-line to the throne. It only makes the security more challenging.

"Unless a sterile zone is put in place it's going to be impossible to make the area completely secure. Frankly, the whole thing is a bit of a nightmare from a policing perspective," a source said.

In related news, the Civil Aviation Authority (CAA) has already announced that airplanes and drone are prohibited from flying above the St. George Chapel on Prince Harry and Markle's wedding day. The flight ban was reportedly requested for "safety and security" reasons.