Let’s say that you were a bad boy and got reincarnated as a medieval sapper (sorry!). You have a low life expectancy and a dangerous job—and to top everything off, you don’t have health insurance (this isn’t Soviet Russia, after all). What are you ever to do? If you said “risk life and limb to undermine the enemy fortification!” than congratulations, you are loyal to your lord! But, now you have a job to do…
Medieval sappers in the early middle Ages usually had one of several options at their disposal when figuring out how to harm the foe’s fortifications; if the opposing palisades were wooden, then one method of engagement would be to advise the lord to use enflamed catapult missiles on the wall in the hopes of setting the whole edifice on fire.
During the later High Middles Ages, however, as more and more fortifications began to be erected with stone—think of the elaborate castles typically associated with medieval Romances—this fiery option became less practical (last I checked, stone did not easily catch fire). So what you are, lovely sapper, to do? One word: trench.
Call it an anticipation of World War One is you must, but the fallback option for stone defenses was to dig a trench. This was not like the WW1 trenches, however (so all you conspiracy theorists can put away your magnifying glasses). These trenches were dug right up to the walls and were done under the cover of wooden shielding. Once at the wall itself, the soldiers and sappers would begin to remove the stone foundation of the wall, thus undermining the entire wall section.
Another method used by sappers for stringent defenses was digging a mine. By this I mean a physical, underground mine leading straight up to the wall’s foundations; held up by wooden planks, the sappers, once at the wall, would begin to remove the wall’s base, as they did with the trench, but this time it was to plant and detonate explosive material. Obviously, this method was greatly effective in terms of destroying that blasted wall! But, look out! Be on-guard for an enemy counter-mine!
That is right: as you are underground, mining your way to the wall, the enemy may, in fact, dig a counter-mine in the hopes of literally borrowing directly into your own mine-shaft, thus massacring you—loyal sapper—and securing direct access to the sieging camp. Damn. But, to give credit where credit is due, such effort would be foolish not to be undertaken by the enemy since if your foe—you!—is digging a mine, they could borrow underneath the wall itself and straight into your castle’s courtyard, thus giving them—you— a platform for raiding. That will not do! So… mining, yeah.
Usually, my dear sapper, you would start your mine at a distance from the castle. After all, advertising to the enemy what you were doing is an ill-advised tactic. But they would have determents of their own—like the all mighty water! Yes, to detect your mining activities, the foe would use bowels of water as seismic devices; think the terror of the glass of water from Jurassic Park, but with more smelly young men hanging out in barracks all day. And no, you have no technology capable of counter-acting this ‘water bowl.’ Next time you are in the king’s court you will have to tell those poindexters to get their booties moving!
Until then, you have a wall to collapse! Get to it!