Was there a Metzudat Tzion written on the Torah? If so, does anyone know if it is available online or for sale?
I always loved this commentary as it explained the hard words on the Nach.
It was written on all of the Prophets and the Writings with the exception of Ruth, Lamentations and Esther. It was not written on Chumash.
No edition of the Torah I have seen has included the commentaries M'tzudas David or M'tzudas Tziyon, but that does not prove anything. However, the author of both, in his introduction says the following, implying that the commentary is written specifically on (and beginning with) the N'vi'im (text is from this paper on the commentaries and the translation is mine):
על כן יעצוני כליותי ללקוט שושנה, לאסוף אמרי"ם מכל הספרים הקרוב לפשוטו ולהציג לבדנה, למען ירוץ בו קורא אחר קורא ושונה, בלי כשלונה, ולהבין דברי הנביא על נכונה, אף בלכתו בדרך ובמלון יחנה, וקראתי הפירוש מצודות דוד
... Therefore my innards compelled me to collect a rose, to gather sayings from all the books that approach simple explanation and to expose only them, in order to encourage one to read and reread [them], without impediment, and to understand the words of the prophets properly, even on the road or staying at an inn, and I called this commentary Towers of David...
Please note, although not relevant to the question, I have taken part in an unfortunately common printing convention of rendering flatly lines which were written in verse. As in many introductions, the lines in the above snippet all rhyme, ending in "נה".