It depends. Entry is primarily a battle of wills and power... for instance, with the Prince, I was only permitted entry with his permission, because his realm was being supported by the Blue Rose. That gave him the power to be far more protected than a single fae usually would be.

Accessing a realm comes in two parts - finding its 'location', or 'frequency' and then opening a gateway for yourself. The latter can be done by force, or entered freely if its left accessible.

It's much easier if you already have a connection to something inside of it... a foot in the door, as it were. That's what happened to the carnival when we were attacked by Morningstar.