The main advantage is that, as a fortress surrounds a contained area and you’re outside it, you’ll always be able to virtually encircle a portion of it if you’re prepared to dig enough. Once you’ve drawn parallel with a section of the defences, you can send a cannon ball skimming along the inside of it to sweep away defending batteries.
This works both ways, however. Approach trenches must progress forward in crab-like sideways moves, never heading straight at the fortress or else the diggers in the trench will find the defenders sending shots down the line of their works.
Plan View - the digger can safely move forward on the left and right, but anywhere in the central triangle means his trench can be fired down.
The result - A plan showing typical trench lines towards a fortress, always staying parallel to the walls or angled carefully away
This digging can only go so far however. As the approaches near the Covered way, the defender won’t be simply sitting on his hands. Sorties can be made to disrupt digging and collapse trenches, while the defenders can also dig their own trenches (‘counter-approaches’) outwards, producing a WW1-style maze of trenches before the fortress walls as each side battles furiously to aid or impede the attackers’ batteries.
Section - The besiegers dig forward
Ultimately though, once the protecting bastions and ravelins are flanked and battered down, with the trenches as near as they can go, the attackers will need to storm the Covered way. One tool of this job is the ‘Gabion.’ Basically a wicker cage like a barrel open top and bottom, this can be thrown down and filled with earth to create a makeshift barricade. When properly backed up and staked into place, this creates a proper trench-like barrier. Assaulting infantry that take the covered way can carry gabions along, throw them up as a defence, and then shelter behind them. This is sometimes referred to as ‘crowning’ the Covered way.
Section - The Covered Way is Stormed
If, by this stage, the defenders are too weak to drive the attackers off, the guns can be brought up to the Gabion-wall and begin firing on the actual fortress wall. Once the wall is smashed down enough by this point-blank battering for soldiers to scramble up the rubble and into the gap, an assault is staged (usually only after a polite invitation to surrender with honour, once the breach has been made.)