28mm Sappers. Sappers were combat engineers with extremely specialized tasks who's methods of accomplishment varied throughout the ages. Trenches were used in medieval times and prior to assist soldiers in getting closer to the wall they would assault safely and under cover from archers. These trenches would be dug diagonally, and often used in tandem with mantlets for more cover.
These sappers would dig underneath walls while simultaneous reinforcing the tunnels so they don't collapse over them while working. Then when ready, prior to the use of gunpowder as explosives, they'd remove or compromise the supports within the tunnel directly underneath the walls and collapse the wall. How often this was used I don't know, but enough for it to be known as a tactic across Europe, Asia, the Middle East and North Africa.
These sappers are provided in full bodies, and half bodies to simulate them being in the trench. Of course you can cut the full bodies however you'd like to simulate different depths
