What is a gopuram in ancient architecture

A Gopuram is a monumental tower, usually ornate, at the entrance of any temple, especially in Southern India. This forms a prominent feature of Koils, Hindu temples of the Dravidian style. They are topped by the kalasam, a bulbous stone finial. They function as gateways through the walls that surround the temple complex. The Gopuram's origins can be traced back to early structures of the Tamil kings Pallavas; and by the twelfth century, under the Pandya rulers, these gateways became a dominant feature of a temple's outer appearance, eventually overshadowing the inner sanctuary which became obscured from view by the gopuram's colossal size.