Torah Temimah citing Pesachim 87a #1: It refers to Torah.

Torah Temimah citing Pesachim 87a #2: It is Keneses (the congregation of) Yisrael.

Rashi: I am strong in love of my Beloved.

Seforno: The congregation of the commoners answers, I tried with my deeds to be a wall that protects.

Malbim (Melitzah): The Ru'ach answers, I was strong against the Yetzer ha'Ra like a strong wall. It did not rule over me.

Malbim (Mashal): The sister answers, I am closed like a wall.

Torah Temimah citing Pesachim 87a #1: It refers to Chachamim (they nurture Talmidim and protect the generation like towers).

Rashi citing Pesachim 87a #2: They are Batei ha'Kenesiyos and Batei Medrash. Yisrael suckle from them Divrei Torah.

Seforno: Chachamim who give rulings, are like a strong tower - a refuge from every affliction, via their Tefilah.

Malbim (Melitzah): Breasts represent the powers of desire and of anger. They were strong like towers, i.e. the troops of the Yetzer, which fight against the city to conquer it, did not conquer them.

Malbim (Mashal): My breasts are as hard to conquer as strong towers.

Torah Temimah citing Pesachim 87a: Yisrael is to Hashem like a Kalah who was found Shleimah (perfect) in her father-in-law's house, and is constantly running to her father's house to recount her praise.

Rashi: I will be like a Kalah who was found Shleimah, and finds Shalom in her husband's house.

Seforno: I was like one who has Shalom, even at a time of affliction. Therefore, now we are worthy to merit the coming of our Mashi'ach!

Malbim (Melitzah): I will be in the eyes of the body like one who finds Shalom - as if I have Shalom with the Yetzer ha'Ra, to the point that it does not fight with me, of I drew it to Avodas Hashem. It made a Bris with me, and "my heart is hollow inside me" (Tehilim 109:22; there is no Yetzer ha'Ra in it).

Malbim (Mashal): Until now, [ha'Melech Shlomo] considered that I did not develop yet, so he did not touch me.