Something I came across in the library stacks the other day:
Castles and Fortified Cities of Medieval Europe: An Illustrated History
Jean-Denis G.G. Lepage
McFarland & Company, 2002
The sub-title of this book does not do the volume justice. This is the most copiously illustrated work possible with clear, black and white line drawings by the author on virtually every page. The text is general-level and appears to be aimed at school-age children or those with little or no knowledge of the time period.
Anyone looking for inspiration in building fortifications will find something to pique their interest. The wide variety of fortifications covered from the 5th through the 16th centuries, urban and rural, wood and stone, make this a volume worth checking out of your local library.
My one complaint would be that there was not much about "Fortified Cities" themselves. Perhaps the title was simply ill-chosen as it would have been more accurate to call it: Castles and Some Urban Fortifications. OK, maybe a little unfair when the author was trying to cover so much and has in fact packed a lot of great material into a very accessible package.