Can I still ambush cards? Yes, because card effects only affect cards in play unless otherwise specified. So Fortified position does not blank characters in hand, so yes - you can ambush a card. For the same reason, Sansa Stark will come into play knelt during a Fortified Position turn.
Also note that Fortified Position blanks all printed text, except traits, but including Forced Reactions. So you can ambush Gold Cloaks and then keep them.