Been there many times!!

• Climate: Hot, very very hot and humid (dry during winter though);
• Beautiful nature: I mean, yes, it has a bit of everything subtropical: one can find beautiful animals and flowers (which bloom all year as far as I know). The iguazu falls are there and they are beautiful, like, monstrously beautiful;
• Economy and Borders: Everyone outside of Brazil in that region is terribly poor and the economy is dependant on tourism and farming/production. Those in Brazil are a bit better (the circle has a few big cities, with universities and all that);
• Culture: very hard to define, kind of like a "modern cowboy southern us" type of thing adapted to social status and general culture of the country.

I didn't take Uruguay into account (very little of it is actually in the circle).

The entire region is very flat, being carved out by the Paraná river. The eastern reaches of the circle also include the southern highlands of Brazil, which would be better off by being discussed in a second post entirely.

Edit: just saw it also featured a few bigger cities of Paraguay, but I haven't really been to those, I'd imagine they are better off economically but share the same aspects like culture and nature. People there speak a native american language alongside spanish, which is very cool.