We visited on a clear night last summer and had a lot of fun. There isn't really much to see at the castle itself, but of course it offers amazing views of the city and the Julian Alps. We rode the cable car up, which in and of itself was fun. It's a very quick and steep ride, and it's cool to see the tops of the buildings as you rise above. i don't remember how much it cost, but the ride was pretty inexpensive.
At the castle, there were one or two free, not-too-exciting rooms to see. There was also an exhibit of photographs by Steve McCurry (famous for the photo of the Afghan girl with piercing eyes on the cover of National Geographic), but we didn't want to pay to get in. There was also a restaurant or cafe or something, which we didn't eat at because we'd heard it was expensive with just average quality.
The star attraction was the city view! We spent quite a while looking out. The viewpoint appeared to have been recently renovated/restored and was quite nice. there was a (free!) WC on the bottom level, also nice. We went up around sunset, so the views were especially amazing. After getting our fill of the view, we explored the outskirts of the castle. This was also where there was a path to walk back down (we opted to ride the cable car). It was a well-marked path of wooden planks/stairs and didn't look incredibly long.
* If you like my review, I appreciate you clicking the 'helpful' button; those of us who take the time to write reviews live for those and it motivates us to do more :) *
If you own or manage Ljubljana Castle, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.