Awesome. Thoses huge Weapons rock. Very well designed, rich of Detail.
Might get too much Polygons fast, but i think, those are too big and too heavy for Tanks anyway. 3x3 Tiles are fine, i would say, so how much Life and Defense will it have? Should Fortesses be harder to destroy than Walls? I think so. And If you do too, they should be comparative more expensive.
About the Problem with the Weapons firing it's own Fortress: How about making them lift up dynamically? Then they might see the Enemy and fire it. They would still be able to fire the Fortess though, but well.. One little Problem lesser.
I like both of your Fortesses. And i would love a round one, which has an "Assembly Line" with the Turrets on it and which can lift up and down, so Weapons could fire in a 360° Angle. I wish i could show you, how i mean this.. :-S
I wonder, what comes out of that Door...
EDIT: Is it possible to design "Multi Building Structures"? I mean, if a Structure can have more than one Ability. Every one of those Turret-Places of this Fortress could then have it's own Turret - you just click on the Fortress, then you see the 3-4 or whatever Turret-Places and can give it a Weapon.
Or you can click on every Turret-Place as it's own, like the Fortress would be more than one Structure. This would allow Fortresses to grow up with your researched Technology. You can switch the Weapons of it.