Agra fort is a UNESCO world heritage site. With or without this distinction, it is a landmark monument in the history of India. It was built by the Mughal emperor Akbar in 16th century, and is still an imposing structure after four centuries. It is made of red sandstone, and is actually a small city in itself - a complex maze of buildings, chambers passageways, towers, balconies.
Today, quite a large part of the fort are out of bounds for the visitors, but the part you are allowed to visit is huge enough to tire one's feet but not the eyes!