//edit: Issue in action:
This kind of problem does seem familiar. I have seen this kind of thing before. One thing I'd be suspect on is the wall mesh casting shadows onto itself. With the extruded wall segments that jut out from the wall, are those separate meshes or are they part of the same wall mesh? what I'm wondering is if the shadow calculations are getting confused about how to cast shadows on itself. If they are try separating those out on their own. You could also think about disabling receive shadows on just those walls, it obviously depends on how you want them to appear but there's a chance they don't even need shadows and the look you're after can be faked (just for the flat walls, not the parts that jut out).

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I've been programming for 15 years building many many games over the years, Agent A is by far the biggest project I've worked on however. Growing up I was in love with games of all sorts, NES was my first console but got my first taste of games on Atari and Arcade. I got lost in PC games all through the 90's and kept up with console games ever since as well. Some of my fave games of all time: