Why should they? In real life they aren't either! Whenever the terrain around the city allowed it, cities would build them lighter, lower or otherwise less expensive (or even not at all). Don't forget that walls were extremely expensive, so when swamps, lakes, rivers, cliffs, canyons, mountains, etc. didn't oblige heavy walls, they wouldn't be build. When cities didn't have enough money to maintain the walls in perfect condition, it could lead to deterioration of the walls, bad reparations, etc. Walls were seldom build at once, every-time a city expanded the new quarters would need new walls.sir aleks the bold wrote:(...) since walls should be the same color and style (...)
There are many reasons why the walls of a city could have variations in color, thickness, height, etcetera. Even the wall standard AK_Brickster suggests would almost be too uniform. However, some general standards can be very useful, like the style of the architecture (Middle-Eastern is highly unlikely in Garheim for example) and the materials the walls are build with (loam/stone bricks/stones/wood).
I think it's in the purpose of the game to limit builders as little as possible. Too much standardization can't be good, can it?