Quatro large blocks iced with a top battlement layer of Duplo, giving excellent firing positions. The "gates" were heavy duty Quatro and remained firmly closed. However the orange inner gates could be raised from inside in a 'marvelous feat of engineering' which defines the everlasting appeal of Lego across the ages (see below):
What became of my Maginot Line fortress guarded by Airfix warriors? (see below):
On their return only my children saw the the finished product (and briefly at that) as my youngest son (aged three at the time) did what all three year old's who have seen the "Hulk" in "The Avengers", would do!
Yes, It lasted but seconds however it apparently took my wife over twenty minutes to clean up afterwards! She was not too impressed at my feat of engineering!