That's cool 🙂 - yep it was but I remember someone in one the books saying somewhere (I think it was Lupin) that you didn't need walls and water to keep the prisoners in because they were so lost in their own personal hell/torment. That would mean that even though there were many dementors there, they still wouldn't need to be so strict about guarding each and every prisoner - hence the security wasn't that tight so to speak. They probably came and went checking on prisoners according to a timetable rather then watching over them constantly.
Plus, once Sirius was in his animal form and passed the first dementor as long as he was extremely careful and focused on not thinking lol (like a human that is) he could make it out because they have a harder time sensing a dog.... and, if he was only passing sinlge, pairs or small groups of dementors he'd still have a better chance then when he was caught in the first place by loads of them. Also, he'd been in there for 12 years, in order to stop going mad he must have learned to suppress his emotions.