While Princess (Twilight Sparkle) may have Spike, her best friends and a responsibility to Equestria and her fellow Princesses in the day...
Twilight Sparkle (Princess) has a castle unexplored and unmapped in the night. Is it really surprising that she would take to wandering her halls? Keeping each and every secret she finds to herself?
Until she finds the very last step, Twilight can't help but return every night.