My favourite in Halifax is probably the Public Gardens
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It is full of 19th century features and rare plants and is right in the middle of the city.
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(my photo)
The Commons are more utilitarian parks and public facilities (tennis courts, etc.). This land was set aside for public use in 1749; in the early years it was used to graze livestock, provide camping grounds for visiting soldiers, and provide a clear firing area from the Citadel to make it easier to defend the city.
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Point Pleasant Park is another nice one, a rough analogue of Stanley Park in Vancouver. It is mostly forested, with some monuments and a mix of 18th and 19th century fortifications.
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