This is a list of fortification related sites that I have found on the web. It is not complete by any means, just a listing of sites I have found interesting. I will add more links as I come across them.
The Coast Defense Study Group Homepage
Fortress Study Group - FSG
Fort McRee "Virtual Tour"
Fort MacArthur Museum main page
North American Fortifications Page
Portsmouth Harbor Fortifications
Army Ground Forces Association--Includes Coast Artillery Living History Units!
Fort De Soto
Camp Hayden
Fort Adams
Fort Delaware Society
The Patriots of Fort McHenry - Table of Contents
Casco Bay Online: Fort Gorges
Fort Hancock
Kodiak Alaska Coastal Defense
Sitka's Coastal Defenses
Fort Saulsbury - Delaware
Fort Tilden History
Fort Baker
Corregidor Then and Now
Fort Macon Ramparts
Fort Rodman
Civil War Field Fortifications
Forts, Tourism, Historic Preservation
CoastDefense Listserve
Forteca: Polish Fortifications Journal
Marshal Vauban HomepageThe South Coast Martello Towers
Western Heights of Dover
Palmerston Forts Society
Fort Nelson (UK)
Fortress UK
Fort Gilkicker (UK)
Pillboxes Web Page
Coastal and Harbour Defences of Sydney, Australia
Capital Defence Homepage (New Zealand)
European Fort Photo Album (mostly France, some Switzerland)
The Defence Line of Amsterdam
Dutch Water Lines
Dutch Fortress Museum
Kuivasaari 12" gun turret
The Maginot Line
Halifax Citadel National Historic Site
Goats of Fort Southwick (UK)
Kite Aerial Photography
This page has some great pictures of various batteries around the Marin Headlands in California, taken from a kite!!!
ERH-SHA-WAN Gun Emplacement at Keelung
Boston Harbor Islands Reservation
ParkNet: The National Park Service Place on the Web
Not really a direct fortification link, but they do manage several coastal fortifications in the U.S., such as Forts Sumpter, Moultrie, Massachusetts, Jefferson, Pickens, and others.