Rio Bec buildings often have interior stairways hidden within their towers. These stairways led from ground level up through the tower to an exit on the roof of the building between the towers. Xpujil and Becan both have such stairways. William Ferguson in "Mesoamerica's Ancient Cities" mentions that "the south pyramid at Rio Bec contains an inner stairway and a tomb, making the pyramid functional as a mausoleum", but it is not clear which building he is referring to. John Hagenbuch has attached this photo of an interior stair at the rear of one of the Rio Bec A towers.
The presence of such stairways in Rio Bec buildings makes me wonder what the roof of the range-type building between the towers was used for and whether it was once the stage for some sort of ritual or ceremony.