Thanks, RobA. I had a "wall damage" layer, but there just wasn't enough contrast there to see the damage apart from the stone block. So, I increased the contrast of the damage layer and also made the wall and damage layer more closely match the shade and tone of the rubble, desaturating the greenish color there previously. I updated the image in my original post based on the feedback. Good call!

Sigurd,
Sounds like a great use for it. Very different from my intended use and yet still perfectly valid. I chose to use a transparent background for the stronghold because it makes the map flexible in that it can be dropped into any number of different environments or adventures depending on need. I had originally intended to use this map as a base of operations for the PCs and the expedition company they're with and expected to be able to swap out backgrounds to account for different seasons. I've changed my mind about the campaign and I'm going in a completely different direction, but I now plan to use this map in a different way.

This map design came from an adventure titled "Citadel by the Sea" in the October 1983 issue of Dragon - I have the Archive CDs which still offer a lot of great material, regardless of the incarnation of D&D.