When I decided to work on the temple, I thought I would do some research. I found a book on Aztec sacrifices at Wowio. It is only 10 pages long (an excerpt or short essay from a larger work) but it gave me a few concrete ideas.
Aztec temples are shaped like stepped pyramids (rather than the smooth pyramid we are familiar with in Egypt). I knew they had stairs on the exterior but I found out that while some temples had one set of stairs up the entire length of the western side (the kind I was picturing in my mind, minus the western detail), most temples alternated their staircases at each level. So the stairs on the ground level went up the western side and then you would have to walk around the edge of the pyramid to the eastern side for the next set of stairs and so on. This allowed for a much longer procession than the single set of stairs and was especially grand when the processors went up two-by-two and split to walk around each side of the temple to the next set of stairs.
Adopting this layout of alternating staircases pushes the PCs through each level of the temple (no climbing straight to the top). And, because it is a yuan-ti temple, the stairs will be moulded ramps. I want the temple to be 5 levels tall at least and the edges of the two lowest levels will be wide enough for full-scale combat.
The top of Aztec temples contained two towers up to 50 feet tall that housed, at their peaks, idols of the god or gods to whom the temple was dedicated. Before these two towers was the large stone altar upon which the sacrifices were made. And flanking the sacrificial stone were braziers that were never allowed to die out. Because the Aztecs had so many temples (most not nearly as grand as the one I am making), their cities glowed at night from the many exposed fires at the tops of the temples.
Following the sacrifice, the body was relinquished to a special warrior who would prepare the meat for a meal to be held within and around the temple. There is no problem with yuan-ti eating the human bodies after sacrifice, so a gruesome kitchen is in order. The bones, particularly the skulls, were then kept in special buildings (more below).
Finally, the scale at which the Aztecs performed their sacrifices was enormous. It was rumored that 70,000 people were sacrificed at the opening dedication of a temple in 1492 in a ceremony that lasted for days. While this number may be exaggerated, a Spanish priest who came to the new world counted 139,000 skulls in one of those special buildings near a large Aztec temple. No one can say how long it took to accumulate them, but the general rule is 1 skull per person.
Here I draw from the Sedlec Ossuary in the Czech Republic. In the early 16th century, a monk gathered up the bones of plague victims and those who died in the Hussite war and brought them together in the Sedlec Ossuary. Some time later, an artist was commissioned to use the bones to decorate the ossuary and to fashion them into pieces of art. The masterpiece was a chandelier containing every bone of the human body. There is also the Skull Chapel in Poland (Kudowa-Zdroj, Kaplica Czaszek), where the entire room is covered in skulls and bones with an altar (this time a Catholic altar) at the far end (thankfully the altar and crucifix are standard, rather than bones).
The yuan-ti have an ancillary building specifically for holding the skulls (and maybe other bones) of their sacrifices. I am not yet sure whether the entire building is constructed from the bones of their victims or if it is simply covered with them (and then whether inside or both inside and outside). There would no doubt be a nefarious altar inside as well as the standard evil necromancer yuan-ti and guardians who animate from the very bone-covered walls and ceiling to attack. I do not think I would go so far as to have the entire building animate as one giant skeletal abomination unless I made the entire building out of bone (with no actual wood, stone, or other walls and ceiling underneath).