Absolutely amazing place. The latest (on whether you can get in or not) as at start of October 2018 is that it will be improved by EU funding, but right now, it's officially closed. However, they have kindly built some scaffolding and a metal ladder that allows you to climb up to a window and get in! Clearly they intend you to get in but it's not officially open and there's no one to collect money. There are warnings that it isn't particularly safe, and you enter at your own risk, so you have to be careful of tripping over the many iron bars that stick out of the concrete. But if you are a bit adventurous, this is a FANTASTIC place. Personally, I prefer it in it's raw form before it's been all "made safe" and not really like it would have been. It's nice to see it "as it was", but obviously, don't expect any notice boards telling you all about it, as there aren't any. And the bonus is, right now, it's free entry, which it won't be when it's "improved" - if modifying a historic monument counts as "improving" it. If you are in Sibernik, Go there. And soon. Before they ruin it.
Seen the fortress from up-close on our boat trip to island Zlarin. Looks really impressive from the outside, its history is also interesting. One can also go take a look on the inside by foot from the mainland.