Tamil Nadu is where a full religious literature developed in a South Indian language alongside Sanskrit rather than in translation from it, and that is what makes the region distinctive rather than the scale of its temples, remarkable as they are.
From roughly the sixth century, two groups of poets travelled between shrines composing hymns in Tamil: the Nayanars, devoted to Shiva, and the Alvars, devoted to Vishnu. Their verses are personal, sometimes furious, often addressed to the god in the second person, and they name specific temples, which turned a scattering of shrines into a mapped pilgrimage network. The Shaiva hymns were collected as the Tevaram and the Vaishnava ones as the Divya Prabandham, and both are recited in temple worship to this day, in Tamil, beside Sanskrit ritual formulas.
A theology was built on that poetry rather than the other way round. Shaiva Siddhanta, systematised in Tamil from the thirteenth century, holds God, souls and the material principle to be eternally distinct, with grace rather than knowledge as the means of release. Ramanuja, working at Srirangam in the eleventh and twelfth centuries, produced a qualified non-dualism in Sanskrit that gave the Alvars' devotion a philosophical footing and became one of the principal schools of Vedanta.
The great temple cities, Madurai, Chidambaram, Thanjavur, Srirangam, are the institutional form of the same tradition.