hrm, according to my lectures at university if i remember them right (quite some while ago) here is the theory why the conning tower walls had to be thicker:
1) optimal shape of the pressure vessel is round due to tangential pressure, and due to the impact angle of the pressure force the components of the force get significantly lower value - especially the active component which has lower impact to the affected surface (similar as sloped armor on armored vehicles), with the pressure hull that is achieved as a cylinder, which has rounded ends, which assist at the structure integrity, where welds are critical points - that is why hull is not riveted together, since rivets could get sheared off, compromising the interior. Ideal shape of the submarine would be in this case - round shape, but that would obviously increase drag, not to mention how would be submarine large in diameter...
2) since the conning tower cannot have rounded top, and the joint between pressure hull and the conning tower is basically perpendicular, the welds have to be particularly strong, so have to be walls of the compartment to cancel the pressure effects out. For instance, pressure hull is rounded, and can have for this reason walls thick lets say 20 mm, where the conning tower has to have to get same results that at least doubled, could be thinner, but therefore different type of alloy should be used.
So if i remember my theory right, these are the main reasons why the aft and bow are substantially thinner than the rest (full round shape), main hull sides thinner since they have rounded tips, and the thickest conning tower components, due to flat cap. Okay, no on to the new topic for the conning tower
p.s.: feel free to correct my theory, or add new reasons
