Looks like because of the way you modelled it, some of the vertices are actually duplicated (you have two vertices in the exact same location). The most common cause for this in Blender is that you went to extrude something and then cancelled it by right-clicking. This actually doesn't cancel the extrusion - the vertices are still there. This can also occur if you overuse the CSG commands.
When you select a vertex, make sure the yellow lines leading away from it appear on ALL the edges leading to it. Otherwise, you need to delete one of the vertices and reconnect the faces to the remaining one.
Also, I noticed that you have some faces that are actually inside the model. Not only that, but those faces overlap some other faces. I'm betting that these two things would really mess up DarkSHADER.
Something you should do that would help you correct these errors is removing all the unnecessary vertices. Any vertice that is just in the middle of an edge for no real purpose should be removed to save on polygons and simplify the model. After you delete all the vertices, you can neatly reconnect them using the F key. Make sure you Ctrl-A and Ctrl-N after doing this.
Basically, the point of all this is to make the model into one, clean mesh. A good way to test if you've accomplished this: select a vertex (any vertex). Hold control and press the + key over by the numpad a bunch of times. If this doesn't select the entire model, you still have some problems to work out.
Before: (poly count: 395)
After: (poly count: 99)
Sorry for the long, confusing post. I hope you learn something from it. I've been considering writing a tutorial on this, maybe I should.
Oh, about the curvy decoration over the door. In game models, you really want to use a few polygons as possible. Theoretically you could cut the polies on the curve in half and the player wouldn't notice a thing in-game. As you're making the model, you need to decide which parts of it are going to be seen the most by the player, and give those parts more polies, while the parts that will hardly be seen or seen from a distance can be less polies.
If this is for an RPG game, the player will probably be fairly close to the door overhang. However, it is unlikely that he will scrutinize it or even really notice it, especially with the texture and lightmapping applied. Therefore, I'm thinking you could achieve the same effect with only 3 or 4 rows of faces. Also, because the curve does not curve around the y-axis, you could use only 1 column and not a single change would appear in the geometry.
I've fixed up your model a little (attached). Maybe you can compare the two and see what you did that should have been different. See how this one works out in DarkSHADER.
Edit: Forgot to attach.
