The party ran across an inconvenient stone castle, complete with large moat. But we had a Monk with Way of the Four Elements:
You can use your action to briefly control elemental forces nearby, causing one of the following effects of your choice:
- Create a harmless, instantaneous sensory effect related to air, earth, fire, or water, such as a shower of sparks, a puff of wind, a spray of light mist, or a gentle rumbling of stone.
- Instantaneously light or snuff out a candle, a torch, or a small campfire.
- Chill or warm up to 1 pound of nonliving material for up to 1 hour.
- Cause earth, fire, water, or mist that can fit within a 1-foot cube to shape itself into a crude form you designate for 1 minute.
PHB p. 81
My Monk used this to dig little tunnels from the earth beneath the moat that spread under the walls. Each action, the monk forms a cubic foot of earth to have several tunnels (probably ~1 inch in diameter). The tunnels immediately fill with water (from the moat), so they last beyond the one minute duration.
After a couple of minutes (depending on the exact dimensions of the moat), the castle collapses (or the front wall shears off, or the entire castle tips forward, depending on exactly how we shape the tunnels). The defenders have no way of knowing what's going on, and no way to prevent it.
Does this really work?