Why Build a Second Gate Outside the First? To Trap Whoever Gets Through

A gateway is a hole in a wall and is therefore the obvious place to attack, which is why fortification design concentrated effort there rather than on the walls themselves. Every improvement to a gate makes it harder to force and none makes it as strong as solid masonry, so the alternative approach is to accept that the gate can eventually be reached and to make reaching it as costly as possible. An outwork placed in front of the gate does exactly that, adding a further obstacle, lengthening the approach and, crucially, creating a space in which attackers who get past the first barrier are still outside the castle and are surrounded.

The features are consistent across many surviving examples:

• •An outer gate, then a narrow passage, then the main gate
• •Walls on both sides of that passage, walked and defended from above
• •Openings in the ceiling for dropping and shooting
• •A bend in the passage, so a ram cannot be run straight through
• •Gates at both ends that can be dropped to seal the passage
• •The approach frequently forced across a bridge and around a turn

The turn in the passage is the detail that does most of the work and its purpose is worth stating explicitly. A battering ram is a long heavy beam that must be swung or run in a straight line, so a passage with a right-angle bend in it cannot be used for one at all, since the ram cannot be brought to bear on the inner gate. The turn also breaks up an assault, since a column entering the passage cannot see what is round the corner and cannot bring numbers to bear. And a right-handed attacker turning a corner one way exposes their unshielded side, which is why spiral stairs in castles turn the direction they do.

The outwork was one element in a layered arrangement and the others are worth naming together. A drawbridge removed the approach entirely when raised. A portcullis, a heavy grille dropped in vertical grooves, could be released instantly and could not be burnt like a timber door. Openings in the vault above the passage allowed defenders to drop stones and to shoot downward, and the tradition that boiling oil was poured through them is mostly wrong, since oil was expensive and water, sand and missiles were not. Doors were sheathed in iron against fire. And the whole gate was flanked by towers, so anybody at it was shot at from two sides.

Gunpowder artillery ended the design along with most of medieval fortification, and the reason is geometric. These outworks are tall, narrow and built upward, which is what allows defenders to look down into the passage, and a tall narrow structure is exactly what cannon destroy most easily. Later fortification went the other way, building low, thick and angled so that shot glances off, and the defensive geometry moved to overlapping fields of fire from projecting bastions rather than to enclosed passages. Several surviving examples in England and France were retained as impressive entrances long after their military value had gone, and some were built for that reason alone.

A gateway is the weakest point of any wall, so an outwork in front of it forces attackers into a confined passage that is overlooked, defended from above and sealed at both ends. A right-angle bend in that passage makes a battering ram unusable and breaks up an assault. Cannon ended the design, since a tall narrow structure is exactly what artillery destroys most easily.