Edward Thompson's 1904 diagram of Edifice 2, Chacmultun
"Edifice 2 is a small structure, built, as will be seen by reference to the general plan, upon the same general terrace, but at a slightly lower level than that of the Palace.
Its front, now almost entirely fallen, presents no visible detail of interest. Its central core is so extraordinarily thick and massive that one is almost irresistibly led to believe, with the treasure seekers before mentioned, in the existence of a hidden chamber within its space.
Due investigation proved this to be an illusion. The core, or hearting, is a solid mass of rubble work, large masses of rock being used as material. I believe that this unusually massive core was the preliminary work to the erection of a second story that for some reason was never carried out.
Upon the west side of this structure is a wall completely blank, save for one small window-like opening in the north end about six feet up from the foundation. No stairway exists, nor are there any signs that a stairway once existed at this point, yet the opening give entrance to a long narrow gallery running the entire length of the edifice and ending abrouptly in the sheer inner portion of the outer wall.
This curious gallery has near the entrance a series of steps that carries it up to a level several feet higher than the small opening give entrance from the outside. The walls and roof of this vault or gallery are of the usual style, and well built, though narrow, as will be seen by reference to the plan a sectional drawing (Fig. 5). I can at present assign no reason for the existence of this peculiar vault that begins and ends so apparently without use."
E. Thompson, "Archaeological Researches in Yucatan" 1904: p. 13
Note: Some lovely old photographs taken in 1886-1889 by the archaeologist, Teobert Maler, are available online from the Peabody Museum of Harvard University and show how the building looked before it was cleared and consolidated.