One thing I always recognise when I wander through the thickets of history is that the human race is quite good at digging! From the earliest times we were always digging or raising stones. Roman soldiers would build a temporary fort for an overnight halt, I've always said that the Roman armies are best understood if we regard them as well armed civil engineers. The built walls and bridges on an enormous scale. Look at the canals and railways, sinking mines and generally modifying the landscape, all by human and horse power. So I suppose we shouldn't be surprised at twelve feet deep trenches for drains and sewers.
Thinking back I shouldn't be so surprised. When I was on the 17 Pdrs in my army days we used to dig a gun pit by hand almost every time we stopped and were always digging fox holes and trenches. It's amazing how fast you can burrow into the ground for protection!
Here's part of the Black Dyke on Blacko Hill. It's an enormous earth work stretching away across the hill towards Weets Hill and Gisburn. Dug by hand as a tribal boundary it is still part of the limits of the Manor of Barnoldswick. It dates back to the Bronze Age or even earlier so they wouldn't even have the benefit of spades and iron picks.... You have to admire the old ones! [And recognise that what we see today is only a remnant. Over the years it has eroded and fallen in, originally it would be much deeper and might even have had a wooden defensive paling on top.]