Learn Italian: Il quartiere vicino al centro è tanto fresco quanto la piazza. - The neighborhood near downtown is as cool as the square.

It forms the comparative of equality, meaning 'as... as'. You put the adjective you are comparing (in this case, fresco, meaning 'cool') between tanto and quanto to show that two things have the exact same amount of that quality.

Since la piazza is feminine, why is the adjective fresco masculine?

In a comparison, the adjective always agrees with the first subject you are talking about. Since Il quartiere (the neighborhood) is masculine and singular, the adjective must be fresco, regardless of the gender of the second thing it is being compared to.

Can I use così ... come instead of tanto ... quanto here?

Yes! When comparing adjectives like fresco, tanto ... quanto and così ... come are completely interchangeable. You could easily say è così fresco come la piazza and it would mean the exact same thing.

Why does the sentence use al centro instead of just vicino il centro?