To answer the other question you've asked in your comment - people are usually not different in a typical civilization or state compared to an empire, the difference that does arise though, is that in empires, you will find a number of different cultures and peoples all joined up together in one big empire. This doesn't happen in the case of a civilization or state.
Empire's are huge and sometimes even span numerous continents as they are usually made by conquering others. A modern example can be the British Empire that spanned from the US in the west, up till China in the east. Civilizations and states are usually based around a single culture or a single religion predominantly, while Empires have even countries where the people are completely different. Empires are ruled by Kings and Tsars, Emperors, while states are usually governed by smaller kings or presidents or similar.
The city's public square where the market is held is called a plaza. Plaza actually means the towns square or open space, so that has to be the correct answer.
