This is the 5072nd most frequent Catalan word.
alto
"Alto" in Catalan means "stop" or "halt."
Here, 'alto' (alt) is an adjective describing the height of the building.
Han construït un edifici molt alt al centre.
They have built a very tall building in the center.
In this context, 'alto' acts as an interjection often used to tell someone to stop.
Vaig escoltar un 'alto!' quan travessava el carrer.
I heard a 'halt!' when I was crossing the street.
'Alto' (alt) modifies the quality of the voice tone, highlighting its pitch or elevation.