Vaclávské Náměstí, pronounced “Vatslavské”. If you want to impress your boys, @tassojunior, just call it Vaclavak (Vatslavak), which is the way Prague people call it.
You got Old City right but, sorry, the New City is Nové Město. And the Old City Square, Stároměstské Náměstí (quite a mouthful) is simply referred to as Staromak.