Note : This page was originally published in 2016 and does not reflect the latest information.
SInce the pandemic, numerous changes have occurred to the rules of visits. Visit the below website for the latest details.
The Imperial Household Agency gives out free walks of the Imperial Palace five days a week. You will get to enter through the Kikyomon Gate normally closed off to the public, and have a close look at the exterior of a hall where official ceremonies are held, as well as see the famous Nijyubashi stone bridge from the "other side" (see the photo below) . You won't be seeing the residence of the Emperor during this tour, but this is as close as you'll get without a special invitation!
The tour starts at 10am and at 1:30pm, lasting for about an hour. While the commentary is only in Japanese, non-Japanese speakers are given audio guides. First there is a showing of a video explaining what you can expect to see, followed by one of the security officers giving out the tour using a loudspeaker. Photos are allowed though the staff doesn't like it if you take too much time with them.
You do need to book in advance from the Imperial Household Agency website. http://sankan.kunaicho.go.jp/english/index.html
Up to 300 people are allowed in the tour, which means that you should have a good shot of being able to join the tour as long as the booking is not at the last minute. You'll need to present a printout of your email for verification when you enter.
Note the area covered on the tour is completely different from that of the East Gardens of the Imperial Palace, which you can enter without reservations any day of the week except for Mondays and Fridays.