If the rebels abandoned the newly captured Curin castle and a Balion or GK army at G-10 decided to move to Curin (which would take two turns) would Curin's default garrison reappear (also takes two turns) before the loyalist arrived?
The garrison troops would be there at the start of the 2nd turn. So yes, the loyalists would need to siege the castle to gain control.
So basically rules are processed before turns.
On the same note really the remaining 10 troops have already "left" the castle as they are being joined to another standing army per the Army Dissolvement section under Lordship Armies in the ruleset. So basically there is not an actual "army" present there at this time.
(It says *New but was added on 1-26-12 when I saw the need for a minimum army size.)