Complex sentence: 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.
Split into simpler sentences: The mortar's base was a large steel box. The base 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.