The entry to Agra Fort is allowed through the Amar Singh Gate only. Some portion of this fort is used by the Indian Army. The public access to that part has been prohibited. So you can not see that area.
However as this fort is very expansive and contains many buildings that are worth a visit so you can very well check them out.
On visiting the Agra Fort, you will come across Diwan-i-Am or the Hall of Public Audience. This hall with numerous pillars is said to be erected by Shah Jahan in the year 1628. On heading little forward, you will see Royal Pavilions abounding in elegant mosques including Nagina Masjid and Mina Masjid, palaces like Macchi Bhavan, Khas Mahal, Shish Mahal and Shah Jahani Mahal and the Zenana Mina Bazaar.
On a visit to Agra Fort, you will be attracted to the intricate carvings and the pure marbles that are used in its making. Besides, the breathtaking views of the Yamuna River and Taj Mahal from the balconies of the pavilions will leave you absolutely mesmerized.