I can hobble along in Romance languages with the assistance of my Latin . . . I once had a hysterically funny conversation with an Italian client who had a little English (which is more than I have of Italian) - we wound up speaking mostly in Latin . . . who knew that would ever be useful?
Catalan has many points of similarity to Italian, Portuguese and Gallego in its structure, but obviously also shares some words (not structure) with French, probably because of proximity.
The best explanation I have heard was that Rome, not long before the fall of the Empire, had settled many retired Roman soldiers in the coastal region of Northern Spain/Southern France. Any soldier who survived the very long conscription got his equivalent of 40 acres and a mule there.
By that time, the earlier Latin that was the foundation of Spanish - because Spain was an important and early Roman colony - had changed considerably in Rome itself. The Visigoths swept through in the midst of all this, but for some reason, they had virtually no effect on the development of Spanish, even in areas that were dominated by them for centuries.
However, about the time the newer Latin might have consolidated its influence, the Muslims invaded, thus paralyzing the change in the south and center of Spain. (Arabic languages had only a lexical effect on Spanish, and never affected the grammar in any way.) Hence, you had an infusion of a later Latin that was free to develop in Northern Spain and the border regions, but not in the center and south, which were the administrative/intellectual centers of Spain and hence became the source of modern Spanish.
Incidentally, the modern language that is closest to Latin is Romanian, which has a large number of Slavic words in its lexicon (just as Spanish has Arabic nouns) but little or no Slavic grammatical influence. This is because Romania (Dacia) was very isolated.
Also, Catalan had a significant effect on the Sicilian dialect, because Spain (mostly through the Aragonese) controlled southern Italy for long enough to make a difference. Pretty complicated, no?