How to get back to original spawn when in creative and spawn resets?

Use Minutor, or some other mapping program to work out the coordinates of your epic castle (in Minutor, the coordinates of the block under the cursor are shown in the status bar at the bottom). Then in-game, press F3 to see your coordinates and walk to the coordinates you found.

Then sleep in a bed and reset your spawn!!

If you built it near your original spawn point, a compass will point you there. You can give yourself one in Creative Mode.