The Bhagavad-Gita, and the Mahabharata, among other texts are far, far older than the 1800s. Not only because of textual and chronological evidence, but also because of evidence from far-away cultures where Hinduism spread to, ages ago.
Bali, for example. Or for that matter, the Angkor Vat in Cambodia.
As for the mention of Mughal structures in India being built over ancient Hindu temples, most of these claims are likely dubious.
http://www.flex.com/~jai/satyamevajayate/tejo.html
Taj Mahal - A Hindu Temple-Palace
Excerpt:
The most valuable evidence of all that Tejo Mahalaya is not an Islamic building is in the Badshahnama which contains the history of the first twenty years of Shah Jahan’s reign. The writer Abdul Hamid has stated that Taj Mahal is a temple-palace taken from Jaipur’s Maharaja Jaisigh and the building was known as Raja Mansingh’s palace. This by itself is enough proof to state that Tejo Mahalaya is a Hindu structure captured, plundered and converted to a mausoleum by Shah Jahan and his henchmen. But I have taken the liberty to provide you with 109 other proofs and logical points which tell us that the structure known as the Taj Mahal is actually Tejo Mahalaya.
There is a similar story behind Every Islamic structure in Bharat. They are all converted Hindu structures. As I mentioned above, hundereds of thousands of temples in Bharat have been destroyed by the barbaric muslim invaders and I shall dedicate several articles to these destroyed temples. However, the scope of this article is to prove to you beyond the shadow of any doubt that Taj Mahal is Tejo Mahalaya and should be recognized as such! Not as a monument to the dead Mumtaz Mahal—an insignificant sex object in the incestous Shah Jahan’s harem of 5,000...