I'm merely repeating what can be found in Vauban's work, but if the siege operations are prepared properly:
1. A tremendous amount of fire is actually falling on the defenders.
2. Your trenches offer a very good protection.
I will use the drawings from Vauban's work
to illustrate this.
First you would have a line of long-range batteries that would bounce over the defending walls to kill defenders and destroy their artillery, thus it's "Ricochet" name. Note how the line of fire is parallel to the attacked portion of the wall (enfilade), allowing the cannonballs to ricochet over the parapet.
Then a second line of batteries set directly in the front trenches. The breaching batteries (in red) would aim at the spear of the demi-lune and bastions to create a breach. Vauban recommends to fire the whole battery at the same time and to hit successively from the spear to the shoulders of the bastion. Attacking the spear makes sense because it's obviously weaker both in terms of architecture and defense.
Note the dots between the (C) batteries. They represent mortars.
Now if we have a look at the trenches, we can see that the sappers will build with sufficient protections. They will first put gabions in front of them as they advance behind a mantelet (on the left), then fascines over the gabions, and finally earth over the whole stuff, creating a parapet. The result is that you have a trench that is elevated above the ground. Some trenches called cavaliers
would be made even higher and cover the stove itself. If you look at the pictures above, you will notice some perpendicular black bars along the trenches, those are called traverses
, defending the parallel axis of the trench (just like you would avoid making linear trenches in WWI).
Now if we go back the Badajoz example, it's clear that the British didn't follow Vauban's advices:
1. There was no trench close to the wall.
2. The area was flooded. It seems that they tried to shell the water retention to alleviate the flooding, but I guess that the infantry attacked in the mud. There is no sign of sapper work on those areas.
3. The breaches were made on the sides of the bastions instead of the spear.
4. They don't have any ricochet batteries or close-range mortars. So of course setting up a defense must have been really easy.
5. They attacked from the south-west in open field. I guess that many must have been killed by defending artillery before even reaching the walls.
If you look at this map, you notice the Pardaleras redoubt in the south. I believe that an attack from the south-west would have been more interesting if they had sized this redoubt. They could have taken the Pardaleras and opened fire on bastion 6 from both redoubts, but they only considered false attacks against the Pardaleras.