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 ?