The final steps will be more detail oriented. I added more snow/ice in any area where it would naturally accumulate such as cracks, crevices and over-hangs.  For this effect I created a custom
brush that looks like one highlighted below.

The specific settings for this brush are as follows:

Shape Dynamics:   Minimum Diameter is 20%; Roundness Jitter 75%; Minimum Roundness 20%
Scattering:  Both Axes turned on at 770%; Count 2.
Other Dynamics:  Opacity Jitter 88%, Control set to Pen Pressure; Flow Jitter 85%, Control set to Pen Pressure
Noise: turned on
These settings were used with a size 6 for the custom brush and worked well for the scattering effect of the snow I needed for this step of the painting process.  Try experimenting with your own settings for a different look.

I also used a basic soft edged brush to paint the wave-like patterns present in the ice wall. It was important to do this step on a new Layer because of the layer effects used to make it more realistic.  I first used a motion blur filter with a slight angle adjustment and a distance of 15 pixels.  I then reduced the opacity to 70% and used the smudge tool to areas of the wave pattern that  I wanted to distort further.

The final step is one that really sells the ice effect and is quite easy to do.  I made sure that I was on the original background texture layer and used the Lasso tool to select the portions of the fortress that were behind the ice wall.  Then under the Filers menu in Photoshop scroll down to Distort and select the Glass filter.  In the Glass filter I used the following settings: Distortion 4; Smoothness 6; Texture Frosted; Scale 135%. The last thing I did while the ice wall section was still selected was to apply a slight color and levels adjustment. In the Color Balance Adjustment menu I used -20 Cyan and + 10 Blue in order to give the ice a very subtle blue hue. This effect was a good solution to give a distorted wavy look of something that is encased in ice.

Here is the final texture applied to the Angmar Fortress model.  Both normal texture and with Ice texture are shown.