L L.
Yelp
Praça do Comercio is in the Baixa neighborhood, closer to the Alfama neighborhood side. It is aka Terreiro do Paço (or "Palace Yard") because it was the site of the Royal Ribeira Palace, before its destruction in the big earthquake (1755). This square is on the Tagus river, historically the ceremonial entrance to Lisbon and Portugal, with the Cais das Colunas (two stone pillars in the water by the shore) acting as the official physical markers of this entry.
There is a lot of public transportation hubbing around this square. From many angles it's hard to get a photo without cable lines in it. There are cafes around the perimeter, but not as many as one would think. The square is busy, but there aren't many places to sit, particularly for the number of people constantly milling around the place. The architecture is beautiful, and near sunset, the light is very pretty, especially with the water nearby. We visited during Carnival, and it was very surprising to us that no live entertainment was happening in Praça do Comercio (because it was in many other squares in the city). No street performers, no neighborhood parades, nothing, just loads of people walking around.
One of the oldest cafes in the city (est. 1782) is just off of this square, Martinho da Arcada. Its history reminds me of the George & Vulture in London (Castle Court), where Dickens used to eat. When visiting the square, it is worth a stop at Martinho da Arcada for one of their very Portuguese natas (egg tarts), or pasteis de bacalhau (cod pastries). The touristy beer museum is in one corner, with cafe seating out front. These are the more well-known spots directly on the square.
In the square's center is a sculpture by Machado de Castro, of King José I, and horse, Gentil (a Lusitano). One of the most emblematic features of this square is the Arco da Rua Augusta, a large arch in the center, giving access to the city beyond. For a small fee (or free if you get the Lisboa card), you can climb the arch and look out from the top.
There is a lot of unique shopping, and nice restaurants off of the square. The Mude fashion museum and Cathedral de Lisboa are not far either, both worth visiting (the cathedral is free).
Praça do Comércio is one of the largest squares in Europe, and a definite must-see on any trip to Lisbon, but as mentioned, there is not much done to accommodate public seating, nor a whole lot to do on the square, so unless you go sit on the wall by the water, IMO, it's more of a quick look-see stop.