Royal Wedding Countdown: New Prince William & Kate Secrets Revealed

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The countdown is on for Prince William and Kate Middleton's highly anticipated royal wedding. The invitations have been mailed out for the Friday, April 29 event, and Royal correspondent and William and Harry: Behind the Palace Walls author Katie Nicholl reveals a few wedding secrets for ET!

"[Kate is exercising] a bit of control over her wedding day," Katie tells our own Nancy O'Dell. "[It's] very romantic, and being able to surprise not just William but the rest of the world."

Of course, everybody is talking about the wedding dress and who the designer will be.

"This is being treated as a state secret; it really is being shrouded in secrecy," says Katie. "[Kate's] determined to keep the wedding dress a secret. When she leaves the Goring Hotel, she's having a marquee specially created over the entrance so that no one will get a glimpse of her until she enters Westminster Abbey."

Approximately 1,900 invitations were sent out last week. Among the guests are over 1,000 of Prince William and Miss Middleton's family and friends; over 50 Members of the royal family; over 40 members of foreign royal families; over 200 members of government, parliament and diplomatic corps; approximately 80 guests drawn from Prince William's charities; 60 governors-general and realm prime ministers; and 30 Members of the defence services.

"The wedding is being split into two parts," says Katie. "The majority of the people at this wedding are going to be friends of the couple. … It's not a state wedding. This really is a wedding for their family and friends."

Following the wedding, approximately 600 people have been invited to the lunchtime reception at Buckingham Palace given by The Queen, and around 300 people have been invited to the dinner at Buckingham Palace given by The Prince of Wales.

Watch ET for more Prince William and Kate Middleton wedding secrets revealed!