I've got three tips for you which should help you to improve your skills.
1) Search Google for "the Wadfather" there you should find better Textures.
2) Scale the textures smaller, then it looks more detailed, and you can fit the texture exact to the shapes.
3) Search for pictures and other games, which have a similar style a your's. And then try to copy the architecture and so on.

Hope, this helps.