Q:

What security measures does Azkaban have?

I have been reading a few questions about Azkaban lately. They all made me wonder what security measures are implemented in/around Azkaban.
I wonder because Sirius Blacks' breakout seemed so easy, he could just slip past the Dementors in his Padfoot form ? Were there really no extra security measures besides the Dementors and the surrounding water ?
And the breakout of the Death Eaters. Now I don't quite remember how it's described in the book, I recently watched the movies again and there someone or something blew a hole into the prison walls, that seems a little easy to me. You'd think that the prison walls are somehow protected against destruction. (You can make a locket close to indestructible but not a stone wall ?)
It's often implied that Aszkaban is a high security prison, to me it seems like there's barely any protection at all besides Dementors, who don't seem to be the smartest creatures in the magical world, and a lot of water.
So are there any canon sources about the security measures in Azkaban ?
EDIT for clarification: I know there are locked bars and high walls too as a security measure. I take them for granted cause that's what you need for any prison I think. 
I'm looking for extra sorts of security, like the Dementors and the rough water around Azkaban. Surely there would be more than that and I'm hoping for some canon source on it.

A:

We don’t really know much about the Azkaban defences.
The walls, cells and water are taken as given, but as Lupin suggests, not much else may be necessary:

“The fortress is set on a tiny island, way out to sea, but they don’t need walls and water to keep the prisoners in, not when they’re all trapped inside their own heads, incapable of a single cheerful thought. Most of them go mad within weeks.”
— Prisoner of Azkaban, chapter 10 (The Marauder’s Map)

J.K. Rowling has provided some more information about Azkaban on Pottermore, but it doesn’t go into specifics about defence measures.
It was originally the fortress of the dark wizard Ekrizdis, and it sounds as if he left some pretty formidable stuff there. After he died and the Ministry discovered the island, they found some dark stuff:

Those who entered to investigate refused afterwards to talk of what they had found inside, but the least frightening part of it was that the place was infested with Dementors.

No details are given, but whatever it was may play into the defensive measures. It was originally built to lure unsuspecting Muggles to their death, so I’m sure some of his enchantments were designed to stop people leaving.
There are more hints that the building itself may be partially sentient:

The very walls of the building seemed steeped in misery and pain, and the Dementors were determined to cling to it. Experts who had studied buildings built with and around Dark magic contended that Azkaban might wreak its own revenge upon anybody attempting to destroy it.

Technically that’s still walls and cells, but those still sound pretty scary (if true).

I disagree that breaking out of Azkaban is “easy”. In the two known cases:

Sirius using his Animagus form. The majority of people aren’t Animagi, because it’s pretty difficult to learn and get right. And you’d have to learn to do it before you get sent to Azkaban, which means that Sirius is probably the only person who could have used this escape tactic. Perhaps an oversight in the original design (then again, Muggles don’t tend to turn into animals of their own free will), but hardly a viable or easy option for most prisoners.

The Death Eater breakout of the second wizarding war. This coincided with the mass defection of the prison guards, who may well have assisted in the breakout. Again, not an option available to most prisoners.

And when it comes to conjuring a Patronus, I suspect sheer volume of Dementors would slow any would-be attacker. You can’t kill a Dementor, only displace them. But Azkaban is swarming with them: they bred there for years, essentially unchecked. When the Ministry first discovers the prison, they’re sceptical of their ability to contain the Dementors:

Others were afraid of what might happen to the Dementors infesting the building if they deprived them of their home. The creatures were already strong and impossible to kill; many feared a horrible revenge if they took away a habitat where they appeared to thrive.

If the Ministry can’t subdue them, I’m very sceptical of a small team attempting to break into (or out of) the prison. I think you’d be overwhelmed by Dementors, coming at you from all sides, and all sapping your energy as they do. A full Patronus is powerful, but it’s not invincible. And if you somehow get through the Dementors, you just have to deal with the prison itself (which, by the way, you probably know nothing about and may well be unequipped to face).
Frankly, I don’t fancy your chances.

A:

You mention the Dementors and the water.  But there were also bars.  Sirius mentions them:

I was thin enough to slip through the bars.

A human could probably not squeeze between the bars.  
Also, the prisoners don't have a wand, which would be a crippling disability for many Wizards who are used to do everything with one.

