The ruins of a great Erebuni fortress, very sad that it's not reconstructed. Also there's a nice museum nearby and the landscape from the wall is amazing - you can see the city and mountains from there. Definitely that one of the places in Yerevan to visit.
You can walk around it within an hour. Basically, there are only remnants of walls of the fortress with some descriptions. We thought an excursion would be great. The museum is quite small, takes another half hour
The fortress and the museum look good, but the territory of the fortress looks somewhat neglected. It is definitely worth visiting this place to learn about the history of Armenia and admire the city from the height of the fortress.