Thatbyinnyu Temple
The tallest of Bagan's great monuments is the Thatbyinnyu Temple, at 61 meters (about 200 feet) tall. The temple is just a half kilometer from the Ananda Pagoda, and just inside the old city walls. It was built in 1144 and is the first of the "temple on temple" designs, where there is effectively a second, smaller, square temple built on top of a larger one.
While the entries at the four cardinal points of the first floor lead only to niches, since the core of the building is solid to support the second level temple, the openings of the second level pierce the square building all the way through.
Note: Like all temples in Myanmar, you must remove your shoes and socks before entering the temple compound. Your best bet is to wear a simple pair of sandles, as the Burmese do. Generally, it's considered "bad form" to carry your shoes into the temple. Lockers are often provided at the more popular temples.