You will be fascinated by the huge canon and can get some gorgeous views of jaipur from jaigarh. Many movies are shot here, concerts held here and weddings hosted year after year.
If you are visiting in summers make sure to carry water, a hat and lots of water. It can get very hot. Jaigarh Fort is mostly about the Cannon- Jaivaan and a walk around to see some courtyards, a small museum and a well preserved lawn.
All in all it is worth spending 45 minutes and admiring the Jaivaan.
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For more on Monuments of Jaipur: http://visitingjaipur.com/things-to-do-in-jaipur/list-of-forts-in-jaipur