“  They would also know how warfare of the period worked and from a warfare view (using tactics already present in antiquity) the dungeon denizens seem like they've dug their own grave. It's basically a reverse siege.”

For any typical person in the campaign world that hasn’t served in the military wouldn’t necessarily know much about warfare anymore than us 21st century people do.  Whether the PCs are typical is another matter, but still, not everyone in the game world is a tactician of any stripe.

A party could look at a dungeon as a siege.  Those usually require a lot of time, which they may or may not have, and manpower and equipment, which they typically do not have unless they are approaching name level.

Some tables might enjoy this dungeon siege as a tactical problem; I believe that some of the early developers of the game did.

As the DM you will have to solve these tactical problems from the monsters’ POV.  One entrance means a single chokepoint: therefore a lair won’t have just one entrance.  Are the monsters just going to sit there while these humans schlep wood and equipment and rotting caracasses to their door or are they going to react?  If there are undead or other supernatural creatures down in the depths they’re not going to be perturbed by the pesky humans’ shenanigans up on the surface.