One of the easiest ways to tell if the image is underexposed is to add a grey card with a value of .125 diffuse in front of the camera. If it reads as 99,99,99 in PS that its exposed 'correctly'
Also the normal map. Make sure it has correct gamma (linear) and correct value ranges. Incorrect gamma will cause these kinds of errors. The thumbnail for the normal map looks way too bright. If you look at a histogram, the red and green channels should be 128-255, and blue should be very bright.
You want to look for the brightness value in the color picker, but you can do some simple math to get the value as well. In this case 173/255 = 0.68 or so.
http://xkcd.com/585/ Proving once more that there is an XKCD comic suitable for all occasions. I don't think I actually have anything relevant to add to the thread.
255% more wonky needs to be thrown on that tower ASAP! You had something going in the concept that was headed in the right direction and the forgot to execute it!