I used Photoshop 7. The river and brown background was on the background layer and the buildings was on another layer.
I made the buildings by using the line tool, turning on a very fine grid (10 squares to the inch) and then selecting the "Snap to Grid" option. This eliminated the need to be steady handed with the lines (to keep them straight) and made it fast.
Simply draw the lines along the grid and make various shaped boxes and buildings. To speed things up, if you get a building shape you like, you can copy that and paste another where you want it.
For any other shape (that is not composed of lines) I simply used the polygon tool and made the shape.
Hope that helps you out.
© 1995-2010 Wizards of the Coast LLC, a subsidiary of Hasbro, Inc. All Rights Reserved.