Fortress

From publisher blurb:

A sourcebook for d20 Modern and Haven: City of Violence d20 Modern.Any soldier worth his salt will tell you that any cover is better than no cover at all. It is for this reason that fortified buildings are still designed and built, and that older structures are retro-fitted and strengthened to better stand against modern warfare. It is the intention of this book to give players and GMs a thorough understanding of the uses of modern fortifications, their construction, and how to attack, defend, overcome, or reduce them in their games.