The Western Wall is the last remaining retaining wall of the Second Temple platform, rebuilt by Herod the Great around 19 BCE and destroyed by Rome in 70 CE. For nearly two thousand years it has been the holiest place in Judaism where public prayer is permitted, and it draws visitors of every faith.
The plaza in front of the wall is divided into men's and women's prayer sections. Visitors of all backgrounds are welcome, though modest dress is expected -- coverings are available at the entrance for those who need them. A widespread tradition is to write a prayer on a small slip of paper and push it into the cracks between the massive stones. The notes are collected twice a year and buried on the Mount of Olives.
The wall is open 24 hours and free to visit. Early morning and late Friday afternoon (Shabbat) are especially atmospheric, though also the most crowded. The Western Wall Tunnels, accessible via guided tour, reveal the full length of the wall underground and are well worth the time.