Tessara

A tessara is a hill or mound, or by extension any other prominence, raised platform, or elevated site, consecrated to the gods and used for the conduct of religious ceremonies. Today, they typically have temples or groups of temples constructed atop them, in an arrangement known as a tessaraon. Thus, when people speak of a tessara they usually mean it in the sense of a city’s temple hill or the raised area on which a temple cluster stands, not in the older and broader sense of any sacred prominence. Both terms are mainly used in Orithea and Rhydain, in the context of the temples and cultic sites of the Thaedonu-Vaati (they are also used in lansds outside Orithea proper where Utara-Vaati is the predominant religious practice, most notably in the Sarcian Empire). In Achuirea where the cult of Rhantashannu prevails, temples are not customarily built atop such hills and the term is not used.  
In ancient times, humans worshiping the Thaedonu-Vaati carried out their rites atop natural hilltops or packed earth mounds which were considered sacred; such were the first tessaras. As early humans constructed more elaborate religious sites, the temples they built were likewise usually situated atop a sacred hill; when none was convenient, an elevated earthen motte, or a stone or brick platform serving as a raised foundation, would be constructed for the purpose. 
Today, the tessara upon which a major city’s tessaraon sits is often shored up with massive stone retaining walls around its edges, turning what may have once been a slope-sided earth mound or a natural hill into something almost like a man-made mesa. The purposes of this may be both practical (maximizing the usable surface area atop the tessara, fortifying it against intruders, allowing it to serve double duty as a citadel or part of the city’s defenses) and symbolic (making the holy precincts of the tessaraon distinctly and unambiguously elevated over, and separate from, the profane environs of the surrounding city). This is certainly not universal, however, and many tessaras in major cities are still sloping natural hills, or are multi-tiered with broad flights of stairs ascending their flanks, or are much more modestly-raised platforms that can be surmounted with a few steps. 
In the regions of northwest Orithea known as Enyrion and Khadoa, and to a lesser extent down the Medrenesse and into Kelimarian lands, the practice of building temples atop hills or mounds is replaced with the custom of placing them on (typically) square platforms or artificial islands in the center of a pool or sacral moat. This preserves the same essential symbolic function of setting the temple apart from the outside world, albeit by surrounding it with water instead of raising it up above (although the artificial island rising up from the pool’s bottom could also still be considered an elevated platform of sorts). In these regions, the term tessara is not used, although a temple precinct or group of temples is commonly still called a tessaraon.