The Citadel provides an outlet for excess XP once a class reaches level 50. All XP they would gain goes into the Citadel, and each level provides a choice between several stats. This cannot be reset later (although this feature may be added in the future).
The different slots are:
Offense:
Power : 2% bonus power per point, stack indefinitely.
Starting Mana : 1% to all mana per point, up to 50%.
Damage Bonus : 0.2% increased damage dealt per point, up to 10%.
Defense:
Vitality : 2% bonus vitality per point, stack indefinitely.
Armor : 5% bonus armor per point, up to 250%.
Resistance : 5% bonus resistance per point, up to 250%.
Ice Magic:
Ice Mastery : 2% bonus ice mastery per point, stack indefinitely.
Block : 1% bonus block per point, up to 50%.
Health Bonus : 0.2% extra health per point, up to 10%.
Poison Magic:
Poison Mastery : 2% bonus poison mastery per point, stack indefinitely.
Critical Hit Chance : 0.1% bonus critical chance per point, up to 5%.
Critical Hit Damage : 1% bonus critical damage per point, up to 50%.
Fire Magic:
Fire Mastery : 2% bonus fire mastery per point, stack indefinitely.
Elite Damage Bonus : 0.2% elite damage bonus per point, up to 10%.
Elite Damage Protection : 0.2% elite damage protection per point, up to 10%.
Light Magic:
Light Mastery : 2% bonus light mastery per point, stack indefinitely.
Healing Bonus : 1% healing bonus per point, up to 50%.
Coin Bonus : 0.2% coin bonus per point, up to 10%.
Dark Magic:
Dark Mastery : 2% bonus dark mastery per point, stack indefinitely.
Melee Resistance : 0.2% melee damage resistance per point, up to 10%.
Missile Resistance : 0.2% missile damage resistance per point, up to 10%.
What picks are optimal for each category, and in what order? Should we just save our Citadel levels for the future?