I would advise you to keep to one thread in the future.
Anyway, I think we should look at some basics, here, rather than jumping to fancy-shamcy stuff like textures.
When looking at something 3D from the front, surface curves are apparent.
Let's take a simple item, a cup, and draw it using the same mistakes you made, then a correct one.
The first one looks like an awkwardly cut or bent 2D plane, as opposed to a solid, 3D object. The effect gets amplified when you color with disregard to (or just forgetting about) a good light source.
Now, lets break this down a little, using the help of a reference image*.
The angle is called eye-level because, as illustrated here, it is the angle we see when our eye (or in this case, camera) is at the same level as the top plane of the Cup. The bottom is slightly curved because we see the bottom plane of the cup from above, and so a small amount of logic-based perspective comes into play.
Think about it. You can simplify the bulk of a cup like this to a cylinder, and a cylinder is made up of two circles and the space between their edges.
And even in a very simple cylinder such as this, you can see that curves are formed, pushing outward. There is no point or angle to which you can move a cylinder and have it curve into the shape instead of pushing out. So basically, following this...
The surface lines of your tower would look a little more like this. I wasn't being very precise, so the eye level is somewhere between that bracket... oops.
The next thing here is your choice of light source.
Front-eye level white light source... While not strictly impossible, not very interesting and tends to muck with the solidness. For assets like this, you probably want a light source that's from the upper right or upper left.
I chose upper right. It helps to draw the directions the light is going off in(shown by those childish lines, here). Now, remember, the object is 3D, and anything 3D blocking the light from touching anything else will cast a shadow, e.g.:
On your tower, the roof and roof tiles would cast a shadow, as would the doorframe and the tower itself (that last one not so much of an issue if your tower is an asset for a cartoon-y game, however).
Another note on lighting, day time light and highlighted areas tend to be warm colors, while shadows tend to be cool colors. This isn't a rule, but it's a pretty good cheatsheet if there's no "interesting" light sources and it's a lit room or outside during the day.
Lastly, I would like to add that all your lines should be about the same weight, or outlines thick and lines inside of those thinner. On your tower, the roof and window lines are thick, while the rest of the lines are thin, and it looks amateurish.
Also, the door needs a handle of some description, and the window either needs to show the inside of the tower, or have some glass in it (I believe this one has already been pointed out enough, though).
The next time you draw an asset, I think perhaps you should draw in the horizon line to avoid slipping up on the surface lines.
Hope this helps!
*Oh, speaking of ref pictures, you should be using those! No tracing, but have a good look for something that resembles what you're drawing and go about studying it! What does it look like from different angles? What's it's texture, and what are its proportions both compared to itself and other things?
Well done with the post. Seriously...