Most likely a porch or walkway attached to a building.
Explanation:
A<em> portico </em>is in fact an extension of a building that leads to its entrance.
The way this is indicated in-text is by describing how the speaker got impressed by the sight of the temple. Usually the first impression is what captures glances of curious people; if the portico is mentioned as a particularly impressive part of the temple, then it must be located at its entrance - that which a viewer contemplates before entering a building.
