Why is this called a Mizmor? It is a lamentation over the Churban!
Rashi (from Eichah Rabah 4:15): "Kilah Hashem Es Chamaso... va'Yatzes Esh b'Tziyon" (Eichah 4:11). It is a joyous song, that Hashem poured His anger on wood and stones (the Mikdash), and did not eradicate His children.
Why does it say "Elokim"?
Radak: He calls to Elokim - how do You hold back that other Goyim entered Your inheritance?!
What is "Nachalasecha"?
Malbim: This includes all Kadosh places - they are Nachalas Hashem.
What are "l'Iyim"?
Radak: They are ruins; a destroyed city is piles of debris. Also "l'Iy ha'Sadeh" (Michah 1:6), its Targum is l'Yagrei Chakra, and the Targum of "Gal'ed" (Bereishis 31:47) is Yegar Sahadusa.
