The Church of Saint Simeon Stylites, known locally as Qal'at Sim'an, is a Byzantine martyrium built around the remains of the monumental column on which Saint Simeon, the first of the Stylite saints, is said to have sat for forty years. The site is located in the hilly limestone massif northwest of Aleppo not far from a Roman road that ran from Cyrrhus (Kyrrhos) southward, intersecting the east-west road between Antioch and Aleppo. The nearest settlement was the ancient town of Telanissos, now known as Dayr Sim'an, which grew with the influx of pilgrims to the site in the fifth and sixth centuries. The martyrium was constructed during the reign of Zeno (474-491). Various buildings were added to the complex during the sixth century. In the tenth century, the site was fortified to serve as a citadel during Byzantine invasions.1

Situated on an elevated hilltop, the church is part of a larger complex that included a baptistery, cemetery and, eventually, rest houses for pilgrims. The complex takes a rectangular form and is oriented north-south. The martyirum lies at the northern end of the complex while the baptistery lies in its southeastern corner. The entrance to the complex was via a monumental gate at its southwestern end which gives onto a "via sacra" leading from the gate to the main entrance of the martyrium in a straight line. 

The martyrium itself is composed of a central octagonal chamber with four arms in the form of three-aisled basilicas extending roughly north, south, east and west. The main entrance to the church is through a gabled, three-arched portico at the southern end of the southern basilica. The east basilica is set apart from the others with an apse in its eastern wall. The remains of Saint Simeon's column are at the center of the octagonal chamber and thus the center of the entire building.

The baptistery is a smaller complex and takes the form of an octagonal chamber housed within a square ambulatory. A chapel was constructed at a later date (sixth century) on the southern side of the baptistery. It's eastern apse is still standing.

The buildings in the complex are crafted from stone and probably featured a wooden roof. Decorative features include Corinthian column capitals and marble columns, as well as carved, beveled moldings that outline archways and arched windows and are characteristic of late Roman architecture in Syria.

The Church of Saint Simeon Stylites is remarkable as one of the earliest examples of Byzantine church architecture in existence. It was reportedly damaged in a Russian air strike in 2016.2

1. Sodini, "Qal'at Sim'an."

2. http://whc.unesco.org/en/news/1499/ 

Sources:

Sodini, J.-P. "Qal‛at Sim‛an." Grove Art Online. Oxford Art Online. Oxford University Press, accessed April 20, 2017, http://www.oxfordartonline.com/subscriber/article/grove/art/T070230.