We come to learn from the Mahabharata and various puranas that Krishna had 16,000 or 16,100 wives besides his 8 principal consorts. Is there any direct or indirect reference in any of the major/minor puranas on how he used to protect his gynaeceum from intruders? In the Mughal period we find that eunuchs or castrated men were deputed for guarding harems. Is there any mention of similar methods in the puranas for safeguarding the chamber of the women? Krishna had to spend quite a long time out of Dwaraka due to the Kurukshetra war and other miscellaneous events. Hence, it seems quite obvious for him to have taken suitable measures to keep his enviable harem, teeming with over 16,000 ladies, away from the lustful eyes of other Yadavas and outsiders. Please do not try to give divine explanations. I want answers strictly from sceptic point of view with authentic reference.
I divided the answer in to two parts: Part I clarifies that the mansions/ apartments of princess/ queens are well-guarded. Part II deals regarding the security of all junior wives of Krishna
In SECTION LV of Nalopakhyana Parva of Vana Parva of Mahabharata, we have some statements in Nala story stating that the apartment of princess/queens are well-guarded. Although it does not answer the question of how they are safeguarding, it just clarifies that the princess/queen apartments at that time are well-guarded.
"Vrihadaswa continued, 'Thus addressed by those celestials, the ruler of Nishadhas spake again, saying, 'Those mansions are well-guarded. How can I hope to enter them?' Indra replied, 'Thou shalt be able to enter.' And, saying, So be it.' Nala thereupon went to the palace of Damayanti. And having arrived there, he beheld the daughter of the king of Vidarbha surrounded by her hand-maids, blazing in beauty ...................................................................................................................................... And Damayanti although herself struck with amazement, smilingly addressed the warlike Nala who also gently smiled at her, saying, 'What art thou, thou of faultless features, that hast come here awakening my love? O sinless one, O hero of celestial form, I am anxious to know who thou art that hast come hither. And why hast thou come hither? And how is it that thou hast not been discovered by any one, considering that my apartments are well-guarded and the king's mandates are stern.' Thus addressed by the daughter of the king of the Vidarbhas, Nala replied, 'O beauteous lady, know that my name is Nala. I come here as the messenger of the gods. The celestials, Sakra, Agni, Varuna and Yama, desire to have thee. O beautiful lady, do thou choose one of them for thy lord. It is through their power that I have entered here unperceived, and it is for this reason that none saw me on my way or obstructed my entrance. O gentle one, I have been sent by the foremost of the celestials even for this object. Hearing this, O fortunate one, do what thou pleasest.'"
Part II :
It is clear from Canto 10, Chapter 59 from Srimad Bhagavatam that Krishna married all the sixteen thousand royal maidens and constantly remained in each of His queens palaces. Each queen had hundreds of maidservants.
SB 10.59.32: Sukadeva Gosvami said: Thus entreated by Goddess Bhumi in words of humble devotion, the Supreme Lord bestowed fearlessness upon her grandson and then entered Bhaumasura's palace, which was filled with all manner of riches.
SB 10.59.33: There Lord Krishna saw sixteen thousand royal maidens, whom Bhauma had taken by force from various kings.
SB 10.59.34: The women became enchanted when they saw that most excellent of males enter. In their minds they each accepted Him, who had been brought there by destiny, as their chosen husband.
SB 10.59.35: With the thought "May providence grant that this man become my husband," each and every princess absorbed her heart in contemplation of Krishna.
SB 10.59.36: The Lord had the princesses arrayed in clean, spotless garments and then sent them in palanquins to Dvaraka, together with great treasures of chariots, horses and other valuables.
SB 10.59.37: Lord Krishna also dispatched sixty-four swift white elephants, descendants of Airavata, who each sported four tusks.
SB 10.59.42: Then the imperishable Supreme Personality, assuming a separate form for each bride, duly married all the princesses simultaneously, each in her own palace.
SB 10.59.43: The Lord, performer of the inconceivable, constantly remained in each of His queens' palaces, which were unequaled and unexcelled by any other residence. There, although fully satisfied within Himself, He enjoyed with His pleasing wives, and like an ordinary husband He carried out His household duties.
SB 10.59.44: Thus those women obtained as their husband the husband of the goddess of fortune, although even great gods like Brahma do not know how to approach Him. With ever-increasing pleasure they experienced loving attraction for Him, exchanged smiling glances with Him and reciprocated with Him in ever-fresh intimacy, replete with joking and feminine shyness.
SB 10.59.45: Although the Supreme Lord's queens each had hundreds of maidservants, they chose to personally serve the Lord by approaching Him humbly, offering Him a seat, worshiping Him with excellent paraphernalia, bathing and massaging His feet, giving Him pan to chew, fanning Him, anointing Him with fragrant sandalwood paste, adorning Him with flower garlands, dressing His hair, arranging His bed, bathing Him, and presenting Him with various gifts.