In South Indian Temples Like Lord Venketesnwara Temple ,Opiliappan Temple ,Ranganathaswamy Temple ,Ramnathswamy Temple. There is arch above the main Deity Idol/Linga.This feature is absent in all temples of Remaining Indian Subcontinent. What is requirement of Arches , and why are they absent in all other temples out of South India.
The arch over the deities is called as Makara Toranam.It is considered to be the guardian of gateways and thresholds.Almost all the deities in the south Indian temples have makara toranam above them.
Wikipedia has a page about Makara a giant sea creature which is also said as the vahana of Ganga devi. The reason of presence of it in south India may be due to the art and sculpture loving rulers in South India who built hundreds of temples during their reign.