Sapiento's Alien Ruin and the use of Sketchup gave me the idea to do this. Sadly I wasn't able to rep it!
I started this tonight and I thought I'd try and go through the entire process - especially the part that involves Sketchup - in the hope that it may be useful to other people out there. Sketchup is a wonderful, helpful, easy-to-use, and FREE piece of the software that can be a great aid on projects like this.
So first a little background - a short story I have been writing is set in my main conworld and is focused around a frontier tower. The frontier isn't so dangerous that it requires a huge amount of manpower or a really long wall - more just a long chain of towers and a few larger forts in the interior that respond to any threat. The towers communicate their status via semaphore flags. Each tower also has a spyglass and can see the flags of the next tower along. So I decided I wanted to draw the tower (it is similar to the old Imperial Border Tower I did ages ago). The tower also has a chicken pen and an outside toilet.
So now to draw it! Well the map part is easy to do but I want to include an illustration - it can add a great deal to a map and is easier than you might think.
This is where I often like to use Sketchup. One of the more painful parts when it comes to drawing things is getting the perspective and the shadows right. You can agonize for ages over this and still not get it right and it will drive you mad.
Basically you can block in the basic shapes in sketchup. In the example below I got a bit carried away but a tall narrow box with a spire-topped octagon atop it would have worked just as well. You will be changing things slightly during the sketch anyway. For a basic house, a cube with a simple gabled roof would be fine. Essentially you are really just looking for the basic structural lines of your building.
Secondly using sketchup allows you to choose your perspective - you can simply pan the camera until you get a view of the structure you like and then just screenshot it before pasting it into your program of choice.
Thirdly Sketchup allows you to include the shadows - and you can adjust the shadows using two sliders to get them just so. I find shadows can be particularly difficult to get right (I didn't use Sketchup to do my recent Forlorn Tower challenge entry and the shadows are something I'm very dissatisfied with).
My next step will be to screenshot the tower below once I decide on a view I like before importing it into SAI (or photoshop) and using it as the basis for my line drawing which I'll post next.