My most common drowning entrance setup is aquifer dependent, easiest to do with a partial map aquifer stuck out to the side somewhere, and i run my entrance hall down alongside it in normal soil/rock.
3 levels. Bottom level normally has an aquifer connection for water control and a descending tunnel to my well system. De-pressurization twist in the tunnel right before the wells.
Middle level is the actual entrance hall which has lever-controlled hatches, that under normal conditions remain open. These hatches are protected by built fortifications, so no building destroyer can get creative.
Upper level has the aquifer source and a series of hatches (also lever-controlled), where the water is dropped down to the entrance hall level and the bottom level.
If the top aquifer source isn't available, a small two pump stack, and a series of water wheels on the entrance hall level but separated from the entrance way, can supply infinite power due to the water source being the flowing bottom level.
Under normal conditions, mist waterfalls, under siege conditions, the bridges snap closed, the hatches close on the entrance level, and the entrance fills up. When everything is terminated, the hatches open, and everything stays sealed as all the water drains back into the aquifer.
Then I go see what I got for Goblin Christmas.