If I play my Darksteel citadel last turn, does my {Darksteel Citadel} has summoning sickness if I {Ensoul Artifact} it?
If you controlled it at the beginning of your most recent turn then it doesn't have summoning sickness. If you cast {ensoul artifact} targeting {darksteel citadel} on the same turn that you played it then it does have 'summoning sickness' and therefore it can't attack. Furthermore, if it turns into a creature on the same turn it came into play then you may no longer tap it for mana that turn either, as it is now a Land Creature, similar to {dryad arbor}, or in this case, an Artifact Land Creature.
Thanks for your help!