14, SPIA, Design a halftrack with front wheels and rear tracks, add an armored roof that can be withdrawn to the sides of the rear compartment to provide cover while mobile and additional armor when in use.
Attach an indirect firing artillery piece that can be adjusted to a high enough angle to use against aircraft also make it capable of firing a 160 mm HE shell and a 160mm varient of the 40mm AA time fused shell.
Built on the heavy tank chassis with 2 170kw gas engines this large spg has an unusual design that has added considerable weight and problematic components. It as 30mm all around armor sloped at 15 degrees, made from 40 tons of steel. Plus an 8 ton 160mm cannon.
The complex transmission that syncs both engines is prone to wear, resulting in frequent breakdowns, made much more frequent by the shear mass of armor it is now carrying.
The barrel of the Gun is covered by large heavy armored doors that take a great deal of effort to open, resulting in dramatically increased time to deploy and prepare to shoot.
These doors require at least 10 additional crew to operate who do not have seats in the vehicle.
I have no idea how the SPIA ended up that way, other then the initial flaws from having almost no good engineers.
It shouldn't be anywhere near that slow, or that heavy, or take that long to set up and the doors taking 10 people to open makes no sense either.
It's ended up as a total new design from what I was doing so I agree to scrap it since my design is already gone. ))
That is, quite honestly, pretty much what you wanted. A heavily armored self-propelled gun with additional armor over the gun which can be deployed to the sides. It's basically something like the StuH in super heavy.
However, I think we can rescue the design for urban warfare. Bolt the doors in the open position permanently, a bit more protection over the main weapon, and it can (due to the angling) destroy occupied houses really, really well. Nothing to improve yet (or at least as a priority), but maybe to keep in mind for later.