What about a game like Worms or Tanks? Simple artillery firing with destructive landscapes. Maybe two castles firing siege weapons at each other?
Worms: http://ciaranmccann.me/wormsjs/
Tanks: http://www.addictinggames.com/strategy-games/tanks.jsp
Worms: http://ciaranmccann.me/wormsjs/
Tanks: http://www.addictinggames.com/strategy-games/tanks.jsp
Sounds like Castle Storm [1].