Scotland defeated Venezuela in their first match since the World Cup play-off loss to the Republic of Ireland.
Pedro Martinez Losa's side led at the half by two goals.
Castellanos was able to catch out Fife with a shot from near the halfway line.
The Scots overcame Venezuela to win.
Scotland's fourth victory in six outings sent Venezuela to a fifth consecutive defeat.
The South Americans lost to Panama on Thursday and the Scots beat them the next day.
Both sides looked to put behind them the disappointment of missing out on next year's World Cup finals as they made four changes to the side that lost to the Irish at Hampden.
Venezuela looked comfortable on the ball as they beat a side who would have taken them to the inter-continental play-offs.
As the Scots began to dominate, they looked to have taken the lead, but as the ball trickled over the line as it emerged from a group of bodies, the referee ruled for a foul.
That was one of the few chances created, despite a wealth of possession, until Celtic defender Clark connected with the ball after 36 minutes.
Emslie headed powerfully home from 12 yards after Brown broke inside her marker and found her teammate.
It looked like a routine win was on the cards until Castellanos showed the class of a player who made the opposite journey to Scotland.
Fife launched a shot over the goalkeeper's head from just inside Scotland's half after his strike partner spotted him drifting to the edge of her penalty box.
After looking ragged and rattled, Scotland regrouped and threatened again at the other end, but the closest they came to making the score more comfortable was a stabbed effort by substitute strikers.
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