Now I didn't read absolutely all of it, but most.
As have been pointed out, at least somewhat, is that the Lizardmen make use of the jungle as defense. I agree with the Lizardmen most probably having walls, but also agree with Scalenex, in that they would probably not want to fight in their cities. What I do think the walls are used for, is to keep dangerous fauna out. The dangerous fauna and jungle in general, is then the actual "walls" of the Lizardmen, in terms of defense against intruders, it is a very wide "wall", but very defensible, manned and patrolled by skinks and all the dangerous fauna.
Hexoatl might not have as much jungle wall-defense, and I guess that is why Mazdamundi created that mountain range, to make another type of "wall", to defend against the Dark Elves. On that note, I do not think Malekith would be able to take Hexoatl. He might be a threat, but Hexoatl has the greatest living mage at its disposal and the Lizardmen's best general, so I think not. Besides the Dark elves can pretty much only get there through a single path, which the Lizardmen guard (as far as I remember), so the Lizardmen has all the advantages, in that they know when the Dark elves would come, and they can make ambushes all the way to Hexoatl and choose exactly where on the way to engage them, and if I know Mazdamundi right, a large portion of Malekith's army would just be chucked down a big hole in the earth by Mazdamundi.