The roofs were always of iron.

The walls were pierced with loop-holes four feet from the ground, and from four to six feet from one another.

Sometimes stone was used in the construction of these walls, at other times iron.

In the latter case the wall is double, the space of from six to nine inches between the inner and the outer wall being filled with earth.
These buildings stood at a distance of from a hundred to a thousand paces from one another; everything depended upon the lie of the ground, and the means at the enemy's disposal; a greater distance than a thousand paces was exceptional.

They were always so placed that each of them could be seen by its neighbours on both sides, the line which they followed being a zigzag.
Between the blockhouses were fences, made with five strands of barbed wire.