From the Super Mario Wiki
The classic in The Palace
The Palace is the first playable field in both Super Mario Strikers as well as its sequel, Mario Strikers Charged. It is located in the Mushroom Kingdom and has a seating capacity of 15,000 people. The field is made out of grass and is the Mushroom Cup's special field.
According to the North American instruction manual, it was at one point a royal family residence, so it's said to be a stadium with great tradition.