Question:
I go to a public park on Shabbos that has a 5 foot fence around it with open and shut doors for entering. Can I carry within those parameters on shabbos
Answer:
One cannot carry in a public park even though it is en-fenced. Since it is bigger in area than “beis sosyim” (approx 70 cubits by 70 cubits) the Rabbis forbade carrying because it is similar to a public domain.
Sources:
O:CH 358! & Mishnah Berurah 2