The mortar's base was a large steel box that was placed below ground, with its top flush with the surrounding surface, allowing the mortar's muzzle to be lowered horizontal for loading at ground level.
The mortar's base was a large steel box. This large steel box was placed below ground. This was with its top flush with the surrounding surface. The mortar's base was allowing the mortar's muzzle to be lowered horizontal for loading at ground level.