Block D Temple
Description of Remains
The most imposing structure in the lower city is the apsidal temple found in Block D. Its size is striking as the whole complex occupies much of Block D and on its north side spills into Block C.  The extant rectangular outer enclosure wall is approximately 69.4 meters in length and 45 meters in width, and the whole complex is on a platform about 1.5 meters above the street level. Access to the elevated complex is from a double stairway. Once inside, there is the fragmentary remains of two rectangular bases at the west end which Marshall took to be bases for smaller votive stupas.
There are a number of features of the outer enclosure wall which suggest it was an interpolation within the grid structure of Sirkap. On the northern side of the temple complex, the existing structures in the southern portion of Block C were razed and fifth street was moved ten meters to the north. This is clearly seen in Marshall's city plan where structure 3C abuts the enclosure wall at its northeast corner. Structure 3C was there first and part of it was destroyed to make room for the expansion of the temple complex. Similarly, at the east end of Block D structure 2D suffered the same fate. What is left of structure 2D partially abuts the eastern portion of the outer enclosure wall. Within the enclosure—Marshall called the enclosed area the courtyard—earlier structures were also destroyed and covered. Just inside the entrance to the courtyard on the east end of Block D, Marshall uncovered a number of earlier structures which had nothing to do with the temple. Marshall also found "the remains of a small apartment abutting on to the south wall of the court in a line with the entrance of the temple, and of another [apartment] in its north-east corner." To properly cover all these structures, the court platform upon which the temple was rebuilt was raised about 1.4 meters above the surrounding street level by using rubble fill to level out the platform. See Marshall, Taxila, p. 151.
The temple itself, located at the eastern end facing west, measures 39.9 x 15.5 meters, and has three parts: an open-air porch, an antechamber, and a cella. The visitor enters the temple from the small porch which leads her directly into the rectangular antechamber.
Marshall uses the Western terms for these architectural features: porch, nave, and apse. I follow Behrendt is using porch, antechamber, and cella.
At the east end of the antechamber is the ruins a wall which most likely had a pair of large, 4.3 meter wide doors that could be fully opened. 
Cunningham reports finding “a massive iron door hinge, bent for the purpose of allowing the door to be turned completely back against the wall,” see Archaeological Survey of India: Four Reports Made During the Years, 1862-63-64-65 (Simla: Government Central Press, 1871), vol. II, p. 127.
Behind these doors is a circular cella which contains a massive, circular, finished pit in the middle measuring 9.74 m in diameter and 5.5 m deep. The floor of this pit was paved with stones and the walls of the pit showed evidence of plastering. 
For Cunningham’s full description of the temple complex, see his Ibid., vol. II, pp. 127-128.