It is why defence, hitpoints, and Prayer skills are often the ones that are lower levels. It's hard to know which of these skills are the most crucial, so it is up to the player's preferences which ones to keep low and which ones to be pushed to the max.
When making a decision, do not forget that the lower the level is, the more easily enemies you will be able to take on, and therefore, the more successful you'll be.
In terms of the most under-reported stat, it's usually Defence, which is not vital for PVP. The higher the defence level, the lesser is the chance of landing an effective attack on him. However, it does not reduce the damage you're able to inflict.
It is also responsible for your capability to wear protective equipment. You'll be unable to wear a well-constructed helmet or armour without meeting the required level of Defence. This is the most important element since better equipment could decrease the amount of damage taken.
What makes Defence less crucial than ability to deal with damage? It is because, besides the Defence ability, you possess the Hit Points attribute that determines how much health that you possess.
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