Ancient Khasi memorial stones, in the East Khasi Hills, in Meghalaya (India)
I noticed a large number of those small square stone structures (about 20-30 cm side for the center hole).
From reading various old books about the Khasi people ancient funeral traditions, I believe that those square structures are small burial chamber, i.e. small boxes where the ashes of the dead were kept, temporarily before a big ceremony (that did not take place too often) where they were moved into a larger Cinerarium (or large burial chamber), i.e. basically a communal tomb holding the remains of all the Tribe's elders.
For more photos and information about these, go to the Khasi Memorial Stones album (read the album description).