Malfoy's plans was clever(spoilers)

Since Dumbledore put every enchantment on the castle it was basically impossible for anyone outside to enter. But then there is a loophole, the room of requirements. The room of requirements appears differently to each person so it was impossible to control. During one of the early years, Montague(one of Malfoy's friends) got trapped into it, therefore Malfoy realized it was a portal to the outside.