If you are following the major quest line, you will defeat several of the different types of wandering minions in the halls, then you will be given a quest to defeat the 4 princes.
Once you get the 4 princes on your trophy wall you will follow a few tied together run around quests to bring you to the point of being told to meet inside the throneroom. After this you can enter.
You never said where you are, but after you unlock the palace, it should take around 2-5 hrs of play to reach the throne room.
I don't know! The gate is blocking the entrance and there is no other way to get in.
you will follow a few tied together run around quests to bring you to the point of being told to meet inside the throneroom. After this you can enter.