After beating the final boss, can I return to the original castle?

No, you cannot return to the original 'easy' castle.

However, if you find NG+ difficult: Equip grace runes.

Grace runes lower the level of the monsters making them easier to kill.

In the early levels of NG+ grace runes also turn the tier 2 enemies back into thier tier 1 form. This can effectively turn the castle back to it's 'easy' form for a while.

You can edit the save file: 'RogueLegacyPlayer.rcdat' using a hex-editor.

The value to change will vary based on your save data. For me as +2, the value to edit was located on the 5th line (00:00000040) as the 15th value in the row (second last). Be sure to copy a backup and change the value to your desired number of completions (IE 01 for new game+). The value maps to 'TimesCastleBeaten'.

Hope this helps