They aren't fabulous illustrations, as you can see in this image from the generic bandit tower pamphlet. But they get the job done. What you may not realize is that it's a composite image made using two different programs. The main program I use for illustration is Inkscape (and occasionally raster graphic programs like the GIMP or Paint.) But the tower itself, and in this case the cart, were created in a 3d modeling program called Wings 3d. I've been using this for years, and although I'd say I still have a long ways to go, I'm reasonably good at modeling things from scratch in it.
What I do is create my scene and take a screenshot of it, as you can see in this image. In this case, each floor of the tower is a separate object, and the cart, if I remember what I did, is something like six different objects. (There's actually another hidden object because I recycled this file from the CorpseBrood Tower model. It's the weird little diamond magical artifact at the top of the tower.)
I then trim the image so that I can import it into Inkscape. Previously, I was using Paint for this, because all I was doing was cropping the image and setting the background to white. The GIMP is notoriously slow when it loads, so for a quick job like that, using Paint seemed wiser. However, I've started using the GIMP anyways because I always apply filters to desaturate the image, so it makes sense to do this in the image prep stage, as I did for the generic bandit tower.
I do both of these steps a second time to get the profile image that I use for the tower diagrams showing what's on each floor. Finally, I open my Inkscape template I've set up as a standard for my tower illustrations and start editing my new tower. I import both the perspective scene and the profile image on separate layers, then experiment with different filters to get different textures. For the cover illustration, I have a background layer where I create abstract leaf patterns for the trees, and I have foreground and detail patterns to draw other elements into the scene, for example the pathway leading to the door, or the projecting rays and bat-winged silhouette on the CorpseBrood Tower cover image.