Are you going to spend money?

The eigth wonder of the world

This is as much of an important decision as any other in the process.  I would consider my collection to be large by AFOL standards but I still find myself making compromises due to the limitations of my collection.  To build at this scale you are going to need literally hundreds of each element you're planning on using in your castle design.  At some point you will be confronted with a decision to either change your design to accomadate your collection or to spend money obtaining the elements you need.  Unless you have spent an insane amount of time on LDD planning your creation first and have an accurate database detailing every single element you own I garentee you will find your collection lacking in some unforseen area at some point.  At least once.  Nobody likes compromise, especially if you're working towards completing a major project.  Where do you go to by hundreds of the same element without speding a ludricous amount of money on sets?  Bricklink - undoubtably the best place online to easily source your requirements.