Defence

Defence is how often things fail to hit you, and it is the only gate on armour. Every helmet, shield, coif, hat and platebody carries a Defence level, so the skill decides what you can wear far more than what you can kill. It adds to your combat level at the same weight as Constitution. High Defence means fewer hits taken, less food carried and longer trips everywhere else.

Armour is the whole point

Nothing else in the game unlocks armour. A piece above your level cannot be equipped at all, so Defence is the one combat skill you cannot route around if you intend to survive the late game.

Wearing the wrong type for your style costs damage on every single hit, once for each mismatched piece across your four armour slots. The Defence cape softens that penalty but does not remove it.

What high Defence does to a fight

Monsters roll their Attack against your Defence exactly as you roll yours against theirs. Thirty levels over an attacker turns roughly nine hits in ten into about four, which is the difference between a food heavy trip and a comfortable one. Hit chance stops reading Defence past 99, though, so levels above that buy you gear rather than dodged blows.

Armour points are a separate number that divides incoming damage rather than blocking it. An amulet of glory adds 12 of them and an amulet of power 6, on top of whatever your gear carries, so a neck upgrade can be worth as much as a slot upgrade.

Quests

Dragon Slayer wants 55 Defence, 12 quest points and green dragons you can kill, and pays 22,000 back. The Fremennik Trials gives 2,812 with no requirements at all, Nature Spirit 2,000, the Lumbridge Catacombs 500 and the Lumbridge Blacksmith 440.