LONDON -- Prince Harry and Meghan Markle's wedding in Windsor may look like a fairytale, but compared to Prince William and Kate's wedding, it's a security nightmare. Just ask Mark Billingham, who spent almost 20 years with the British special forces.

"For the security forces it really is a massive headache and a lot of planning, and time, effort and cost to prepare for this," he said. "I was thinking, thank God I'm not on duty, to be honest."

Unlike William and Kate's ride to Buckingham Palace, Windsor is a medieval warren of confined streets with row after row of tightly packed apartments, balconies and rooftops that overlook the procession.

"It is a hell of a job to secure," said Dai Davies, the former head of royal protection. "It has narrow streets, cobbled in parts. How do you secure several miles of very small township and parts which are open and green?"

Roadblocks and barriers will stop vehicles from getting anywhere near crowds. Even with more than 100,000 people expected, the commander of security of the wedding assured us every single person will be screened.

"The technology is very different now than it was in 2011," said David Hardcastle, the royal wedding strategic commander. "Everyone who's coming into the processional route will go through airport style security."

Beyond street level, this week police announced a 48-hour no-fly zone around Windsor, and we're told defenses are in place to take down any drones flying overhead.