Tom
Google
Well worth it for a day out, so much to visit from the castle/ fort itself, the various viewpoints, the cable cars, the Olympic park and stadiums, the botanical garden, Catalan national art museum and magic fountain. I didn't actually do the famous cable cars but everyone says they're great, however it is worth mentioning that it isn't just one singular trip that takes you from the port to the very top where the castle is, the one from the port takes you most of the way up but then you have to buy another ticket for the 2nd one to the very top where the castle is. In my opinion if you have already taken the first one from the port you don't need to then take the 2nd one to the very top, the 2nd one is very short and takes less than 10m to walk.
Anyway if you're like me and wanted to walk up the whole thing it doesn't take as long as you would think, there are quite a lot of steps/ steep roads but its very doable if you have no issues walking, if you do have problems with walking then its probably best you look into the buses and cable cars or just take a taxi, also there is an elevator in monjuic castle that takes you up to the main bit of the fort where you can see the amazing views and artillery cannons but there is a 2nd level to the fort which you can only access via stairs but honestly there isnt anything up there and the views from there are the same as the 1st level of the fort.