LUGNETTM (LEGO® Users' Group NetworkTM) the worldwide discussion forum about LEGO® bricks.
If you want to talk specifically about brick wargame go to the gaming forum of LUGNETTM
Brickshelf the biggest collection of personnal LEGO® creation images.
Bricklink the worldwide market place to buy LEGO® parts. You will find here all you need to build your army or decor of your next wargame play!
Brikwars (by Mike Rayhawk) : The Ultimate and one of the first wargaming system for LEGO® bricks (it's on Internet since 1995). The simulation system and the rules are far more complicated than the wargame proposed here, but maybe it's what you are looking for.
Brick Battles (by James Brown): A very simple brick wargame system, its rules take only 2 or 3 pages.
Brick Quest (by Peter F. Guenther): A LEGO® game system made for small-scale dungeon quests. Each player play only one character, so it's more a role playing game than a wargame. A modular building system is also proposed to build different rooms for the dungeon.
Classic Castle: a website about the Castle theme in the LEGO® universe. you should have a look to this website because a lot of brick wargame gamers like to play in the castle theme.
If you want to talk specifically about brick wargame go to the gaming forum of Classic Castle.