Rashi: Sela alludes to helping him at Sela ha'Machlekos, when Sha'ul was about to trap him (Shmuel I, 23). U'Metzudasi is an expression of a fortress.

Radak: I have no fortress to be elevated in it above my enemies, other than Your name - "Migdol Oz Shem Hashem Bo Yarutz Tzadik v'Nisgav" (Mishlei 18:10) .

Malbim: At a time of serenity, no affliction reaches me at all. This includes (a) protection from enemies - Sal'i; (b) A higher level, that he defeats the enemy and rules over him. Sal'i u'Metzudasi u'Mefalti alludes to three levels of protection. Mefalti is salvation from the enemy. Mivtzar and Metzudah is a fortress to which he can flee and be saved. The Metzudah stands on a high Sela (rock).

Rashi (Shmuel II, 22:2): He saves me with the army of Yisrael in war, and He saves me when I am alone, e.g. from Yishbi b'Nov (Shmuel II, 12:16-17).'

Radak: A rock and tower in which a person is elevated, sometimes he is saved in it, and sometimes not; his enemy seizes him there. Your name saved me every time!

Rashi: This is like Sal'i (my Rock). A rock is a shelter for travelers, from rain and wind. Radak (Shmuel II, 22:3) - in Shmuel we must say that it is a noun, for Elokei is Samuch to it. Here it says "Keli Tzuri"; both are adjectives. Alternatively, also there Tzuri is a noun (my strength). This is like "Tzur Olamim" (Yeshayah 26:4) - the strength of the worlds and what sustains them.

Radak: This is like Keli, which refers to strength and ability. Tzuri repeats this, via a different word.

Malbim: When Tzur is attributed to a person, it shows special Hashgachah and clinging, which helps to defeat the enemy. This requires three things - Magini (a shield), Keren Yish'i (to gore the enemy) and strength at the time of battle - Misgabi.

Rashi: I will be covered in His shade. This is like "umi'Bli Machseh Chibku Tzur" (Iyov 24:8).

Radak: Because (He is my rock and salvation), it is proper that I be covered in His protection.

Radak: Verses refer to strength as Keren, e.g. "v'Karnei Re'em Karnav" (Devarim 33:17). A ram, bull and Re'em ¹ gore with their horns; Hashem shields David with His name; it is a horn to gore his enemies and save him from them (this is why it says Yish'i). ²

Rashi (Shmuel II, 22:3): He supports me.

Malbim: He gives to me strength at the time of battle.

Radak citing Shochar Tov: Ten Giborim, haters of David, fell in front of him - Sha'ul, Do'eg, Achitofel, Sheva ben Bichri, Galyus' three brothers and Galyus, and the Mitzri that Benayahu ven Yehoyada killed. Therefore, David praised Hashem in 10 expressions - Chizki, Sal'i, Metzudasi, Mefalti, Keli, Tzuri, Machsi, Magini, Keren Yish'i and Misgabi.