As featured in the both the 99 Attack guide and the 99 Strength guide, the best places to train Defence in order to runescape gold, or any melee skill in Runescape; is either at Giant Spiders or Bandits.
The first place many people train, once they become serious about training to 99 Defence, is at giant spiders. To learn everything you need to know about training at the Giant Spiders training spot, please check out The Giant Spider Training Guide.
Although Giant Spiders are a great way to get easy and fast experience when training to 99 Defence, seeing the same green level in the Stronghold of Security and hundreds of “giant” spiders crawling around may become redundant. Another great place to train is at Bandits.
Training at Bandits does cost a little more than training at Giant Spiders; however, Bandits provide slightly higher and more consistent experience. For all the information required to know how to train at Bandits, please check out The Bandit Training Guide.