Located high above the city, and probably dating back to Roman times, Ljubljana Castle is worth the time and money, just for the views alone. You have 2 ways to get there. One is going up the hill by foot, and the other is by funicular, which costs some euros. There are trails leading to the castle, but they can be a bit steep. The funicular is an easier way, but you do have to pay.
Once there, you have some options. The trip to the top of the tower is enough reason for this visit, for nice views of Ljubljana. But you have to pay admission for that, at a ticket office, and go up some flights of stairs.
Other things in the castle include a restaurant, chapel, and museum. The chapel is worth checking out, and a good place to rest your sore feet. And the museum is short but okay, as it gives a quick summary on the history of Slovenia. And the museum part is only worth going if you won't have time for other museums in the city.
If you have time for only one thing at the castle, it would definitely be the climb to the top of the tower. Worth the time and money.
This review is the subjective opinion of a TravelPod member and not of TravelPod.com.