Indra is a deity very closely associated with Buddhism, the edicts of Ashoka were carved on pillars which took their idea from his staff. The story of Indra goes way back to pre aryan beliefs of water cosmology. The edicts were engraved on pillars which laid the foundation for later styles of pillar architecture.
The buddhist chaityas were the first to display such maturity in style, where the symbolic pot containing the tree of life was actually enhanced into pillars, still retaining its original form and sybolism. Of course along the way this changes but traces of it are seen in this ratha right here at mahabalipuram.
Whats interesting about this ratha is that it takes its idea from a remote chaitya in besra, where the chaitya griha and the vihara have been incorporated into the one single building. Yet when a chaitya griha is removed from the live rock and reproduced as a structural temple, or carved out of monolithic rock, what appears finally is the nakula sahadeva ratha. The barrel vaulted roof from the outside now appears like "gajaprshta" or elephant back side which is very clear in this picture. Hence its also clear with the monolithic rock elephant carved next to it, that the cave has been dedicated to Indra. Whats stands out most is the prominent arch window at the entrance of the ratha, depicting the altar inside, though the real altar is not carved into the rock as its rather small. The pillars of this ratha though are more inclined towards pallava style than towards the old buddhist tradition of pot based pillars. Two lines of haras crown the super structure indicating a merger of two architectural traditions - buddhist and dravida.
It might also be of great interest that the first cave at ajanta actually depicts a small dravida temple from mahabalipuram on its walls, indicating a healthy exchange of ideas between the two regions.