I love some of the brushwork here, especially in the first pic. What's really hurting it, though, is the weak texturing job. I suggest getting a custom texture wad, one that has a complete set of 'matching' textures, to retexture the map.
Holy crap, you aren't using the textures I put on my site, are you? There are some okay random ones in there, but overall they are godawful!
Screenshot 1 - Love the architecture, and the cool crystal. The dark cobblestone texture on the arches, though, looks a little odd. And color-wise, the textures just don't mix too well.
Screenshot 2 - Rock texture on the wall is scaled way too high, and it doesn't quite fit. Maybe if it was smaller and aligned better?
Screenshot 3 - Nice, but it could use some texture or architectural variation at the tops and/or bottoms of the wall.
Screenshot 4 - Like how it looks like you can get up on top of the rock formation. Again, though the texturing is a little uniform.
Screenshot 5 - You are using the same texture on the floor as on the walls, never do that. You might make the stone pillars a different material, too, to add some variation. Ceiling is interesting, and I love the little lantern!
It's hard to tell without a download, but the scale seems like it's a little off? This seems a common issue in castle and temple maps, because they aren't quite normal structures. Part of the problem might be the scale of the textures- if they are blown up big they can make the player's perspective seem tiny.
I'm going to see if I have a texture set for you to try that might work better, just to give you some options.
The wad
Dragoon on
The Wadfather might be useful.
I'd also recommend
Blazeeer's wads, but all the online links to his textures and sites are down as far as I can tell. I combined them all into one wad a while back. It's about 50mb zipped, I'll upload it and provide a link and texture file with credits and links to his old sites. Since there are several differently themed wads combined, you'll see a lot of variety. Many of the wads are sorted by prefix, though, so you can just pop that into the texture browser and there's the whole set right in front of you. It is probably the best set of HL1 textures out there, period. I've used it to texture two or three maps, it's wonderful- everything matches, and he usually provides variants of the same texture so you can do nice floor detailing, etc.
Texture Wad Links
Dragoon (5mb)
Blazeeer (52mb)
Hope that's helpful. I'd strongly recommend the Blazeeer textures to anyone still doing HL1 mapping.