Lighting Issue

Hello again,
Please see image below. The walls you see here are one, duplicated, Prefab; that does not overlap (using Snapping tool). As you can see, some of them generate different shades of the same colour and sometimes they will even have lighter spots on hard edges. Does anyone know how to fix this or have any suggestions on simple lighting? I don't need fancy crap and light probes and fog and all that. I want everything illuminated equally and to cast static shadows. I don't need a moving sun, etc.