The Red Fort of Agra is known by many names, the Red Fort, Agra Fort, Lal Qila, Fort Rouge and Red Fort of Agra. It’s only a short distance from the Taj Mahal so you can easily see both in the same day – its only about 3 kms away and on a clear day you can see the Taj Mahal in the distance from the upper parts.
The fort is actually a walled city and contains many buildings including palaces, gardens, audience halls, and mosques. The buildings vary in age and were built under the reign of different Mughal Emperors.
Many of the buildings are beautifully carved and etched on the stone or marble and like the Taj Mahal, the fort is also a UNESCO World Heritage site.
Emperor Shah Jahan, who built the Taj Mahal and parts of the Red Fort, was imprisoned by his son here in the later years of his life. From his prison, he could see the Taj Mahal, the mausoleum in the distance he built for his 3rd wife.
For more history, read the Wikipedia page.
For visiting, its open sunrise to sunset and entry fee is 250INR / $NZ5 / $US4 for foreigners. For Indian nationals its 10INR / $US0.16 / $NZ0.20.