A good idea is to start with a hike around the Temple for first impression and to observe the outer walls and entrances. There is a little path.
In the case of Beng Mealea (around 1 km2) I recommend to go around clock wise from the main access route, and you will come across the most convenient passage to the inner complex at about three quarters. Expect to take about 45 minutes as you walk slowly around.
The atmosphere is breath taking.
Wikipedia has a drawing that outlines a walking tour, but we added more locations on the east side, back (north) and west side. The beginning of the tour is a set of wooden stairs to the right of the unrestored, impassable south gate. The walking tour mostly follows a boardwalk that was originally constructed during the filming of the movie, Two Brothers, in 2004, and is still being expanded. There are licensed guides at the top of the stairs if you would like to have an escorted tour; however, it's actually not difficult to find your own way along the boardwalk.
From the parking area near the market, you cross a stone bridge over the moat and approach Beng Mealea along a dirt walkway to the unrestored, impassable south gate. There are public restrooms on the right just before you get to the ruins. See also a videoclip.
Most of Beng Mealea is unrestored making it even more interesting. New areas of the temple are still being added to the boardwalk. The boardwalk is definitely not handicapped accessible. It winds up and down with many steep stairs and ramps. It also goes through the collapsed Turtle Carving Library on the northeast side of the inner gallery. See also a videoclip.
The normal way to finish the Beng Mealea walking tour is to head directly south from the Northeast Library toward the Southeast Library, then back to the original boardwalk. In the picture that is through the hole in the wall on the left. Instead, we went over the north outside wall, then west toward the North Gate.
We did not try to go inside through the West Gate. We just went under its platform and continued south. Actually there was a huge building just to the west of the platform. From the Beng Mealea diagram it is the same shape as a structure on the east side. The East Gate was the main entrance in ancient times but is now closed due to dangerous rubble. We did not go to the East Gate, but evidently you can enter the outer gallery through a small door just to the right of the gate and see carvings of the god of fire in the lintel over the first door by the first ramp, and an image of a three-headed elephant.
This building was next to the south outside wall and had some really nice carvings on the three doors facing west. These blind doors, lintels and pediments are the style found at Angkor Wat. Notice the large stone hand or foot lying on the ground by the south door.
Many trees (e.g. strangler figs) grow on the ruins in Cambodia. When they get too large and their weight starts to crack the building or wall, they must be cut down. This is an example on the south outside wall of Beng Mealea next to the three-corridor building. The tree had already been cut down but you can get the idea.
The guide led us from the southwest corner of the outer gallery through a maze of buildings and rubble to another "tunnel," and then back to where we started the walking tour. I am sure glad that I did not have to find the way myself. Also, a mysterious energy tried to capture me in the "tunnel" (see the Warnings and Danger tip).
The east side of Beng Mealea's outer gallery is off the boardwalk so there are fewer people. To get there you must walk along a ledge overgrown with roots. See also a videoclip.
Normally you are pretty much on your own back here. Actually one of the guides was bored and showed us around for free. See also a videoclip.
There is much more to Beng Mealea to the north but we did not have time to explore it. Instead we went west along the back wall.
Inside the collapsed Turtle Carving Library in the northeast corner of the inner gallery there is a large rectangular stone lintel with a carving of the churning of the ocean of milk with a turtle.
From the platform at the top of the steep stairs on the north side of the inner gallery the path goes down and through a dark "tunnel" toward the Northeast Library in the outer gallery.
We just walked around the outside of this corner. It was totally unrestored but had some interesting carvings.