The first good place to train in Runescape for attack, defense, and strength training is giant spider training. Giant spiders are level 50 and are located on the third floor of the Stronghold of Security in the Barbarian Village. The third floor is the bright green colored floor.

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You will need to have completed the first two floors of the stronghold, in order to get down to the third floor to train.
Giant spider training will give you 266 exp for each giant spider you kill, and you can hit fairly high hits on them, as they have low defense. They also have a pretty low attack, so you will be able to train for a good amount of time.
Giant spider training, with 99 Strength, will get around 50k-60k exp per hour, depending on the armor, weapon, and potions used. Speaking of the type of armor, weapon, and potions used, the next section will explain "What to Wear for Giant Spider Training."