Half reclaimed by jungle and gradually being encroached on by villages, Tughlaqabad (adult/child ₹300/free) was Delhi’s third incarnation, built by Ghiyas-ud-din Tughlaq in 1321 against Mongol invasions, and abandoned just six years later. In contrast to the decorative flourishes of the Lodhi Sultans and Mughals who came after him, Tughlaq's taste was for stern and functional defensive bastions. There are fantastic emerald-green views from the high points; interlinking underground rooms, which you can explore, were used as storehouses. There's also a creepy baoli to explore.
The sultan’s well-maintained sandstone tomb (included in the entry ticket) once stood in the middle of a lake, but is now separated from his fallen city by a road. It’s almost as well fortified as the fortress.
It does get hot out here, so bring plenty of water and snacks – there’s nowhere to buy anything. To reach the fort, take an autorickshaw from the Govind Puri metro station.