Nearby, is the "Sun Temple" that was a complex originally very well protected. In Inkan times only the priests and the Inka could use those temples; thus, they remained closed and protected. Common people had popular ceremonies in open areas or plazas like the one in Machupicchu or Qosqo. The entrance into the Sun Temple is through a magnificent double jamb doorway, that on its interior surface shows its security system with a stone ring over the lintel where the wooden door must have been hung, and the two stakes inside small carved boxes in the interior jambs where the door's crossing bar was tied. The temple itself was built over a huge "in-situ" boulder. It has a semicircular floor plan; its rear wall is straight and the whole temple is built with the "Imperial Inkan" architectonic type, that is, with rectangular faced stones with perfect joints. The semicircular wall has two windows; one of them faces towards the east and the other towards the north. According to modern scientists those two windows constitute the most important solar observatory in Machupicchu; in the window facing east it is possible to fix accurately the winter solstice measuring the shade projections on the central rock. Both windows have projecting carved fake beams surrounding their outside face; they surely served in order to support elements that made solar observations easier. In the center of the temple there is an "in-situ" carved rock altar that served to carry out diverse ceremonies honoring the Sun; it is over here where animal sacrifices were executed, so that analyzing their hearts, lungs and viscera, the priests could foretell the future. It is also here where the Inka had to symbolically drink "chicha" (maize beer) along with his father the Sun. The straight rear wall has a window with small carved holes on its threshold that tradition knows as the "Snake Window" (name given by Bingham). The holes are very similar to those found in the Temple of the Stars in Qosqo's Qorikancha that according to Garcilaso kept ornaments of precious metals and stones; possibly also over here those holes had the same duty. The straight walls of the temple have trapezoidal niches in their interior faces; they were used to keep different idols and offerings. Some authors indicate that originally this temple had a thatched conical roof, and they name it as "Suntur Wasi", "Military Tower", etc.