Don't do what I did: come here in winter! It gets snowy in the area. The area is still open though. The main attraction and definitely ONE of Iran's main attraction to see is Falak ol-Aflak aka Shapur Khast Castle which is based on the town of Khorramabad. It makes me wish I had a drone to cover it better, similar to Palmanova. The pictures you can find online are better and definitely something that will inspire you to go. So maybe Iran isn't quite on the "green" list of places to go with their political drama but if you DO make it to Iran then don't just visit Tehran and ensure you visit the castle.

The location of the valley encompasses the entire area. In fact I don't think it REALLY wants to cover the castle as well. It is more about the caves that are not all visitable as far as I understand. Let's first finish off the castle though. Inside, which is mainly nice for the shape, you also gets a museum. It covers the entire valley stuff too so while it isn't going to be major insight into life 40,000 years ago you will learn more about the area and it's surely a must-see.

As for caves, I suggest Kogan Cave. It's a bit south of Khorramabad. It took me an hour to reach the places to park but wait, you still have to hike up to the actual cave! It takes a good 30 minutes and I don't recommend it for anyone with health issues. It was maybe worse in the snow but you don't have a good path most of the way. When you get to the cave it's metal steps to get around. You get to see the cave, all the rooms (like a dozen and some very tall) and most impressive: views of the town including the castle. You don't get to see anything related to the ancient history of the caves here though so that's why I recommended visiting the museum earlier. I felt the cave visit is more of a "tick" but the main attraction was back in town - even if it's not the one inscribed in the end.