"náměstí" is neuter both in singular and plural. The same word is used for both singular and plural.
The ending -é is the normal ending for a singular neuter adjective. The ending -á is the normal ending for a plural neuter adjective.
The ending -á is also the normal ending for a singular feminine adjective. The ending -é is also the normal ending for a plural feminine adjective.
Because your ears aren't tuned to Czech (yet).
The pronounciation of "náměstí" is /námňesťí/ where both /ň/ and /ť/ are palatal consonants. The palatal /ť/ is (in terms of the place of articulation) somewhere between an alveolar (regular) /t/ and a velar /k/. If none of the languages you speak uses the palatal /ť/, chances are pretty much equal that you may hear the sound either as /t/ or /k/, or a "softened" version of them. When pronouncing it, the tip of the tongue needs to be placed behind the teeth, as if pronouncing /t/, and the middle of the tongue needs to be pressed against the roof of the mouth (the palate) as if pronouncing /j/ (or "y" in "yellow") at the same time.