In order for the Demon Castle to take manifestation on to the world of man, there are many things to consider. First we must break down Dracula's “curse.” This curse was designed to drive the average man or woman insane (thus filling their hearts with darkness, cruelty and evil intent). This would cause a tilt in equilibrium (light vs darkness) and this can force the castle to appear, even if Dracula is not resurrected. The other concept is that Dracula's presence must be felt. This could easily be one of the following: Spiritual, Resurrected and or his body parts still remain. Just the overwhelming application of having his body parts, is enough to drive the average person mad (think Maxium from HoD). With this said, just his body parts alone are also able to allow Dracula to possess the weak willed, and darkest of heart. This, too, also applies to Dracula's 'curse', hence man's desire, can also be his darkest intentions, and can be his undoing.
Now another thing, the Demon Castle itself can also appear if “Chaos” senses it's master, since the castle itself is not a building, but a living, breathing entity; a “Creature of Chaos”, so to speak (as quoted by Alucard from SoTN).
So the common factors are -
1. There must be enough “darkness” in man's heart to tilt the scale
2. Dracula's presence must be felt (Resurrection, Spiritual [Possession/Wraith], the Castle senses/beckons its true Master)
So, from source material (manuals, games...etc.) Majority of what above posters stated is true. The castle will appear out of no where, normally some where within the Carpathian Mountains, engulfed from a black mist. Anyone caught around this, are immediately trapped within the castle, and or fall under it's influence (even animals can fall to this; think of the Birds...etc.
Once inside, there is no way to escape, unless very lucky or until Dracula is put back to sleep for another 100 years (or pre-resurrection)