Halifax's downtown has always been drifting uphill and south, and changed suddenly in the 1960's and 70's due to urban renewal projects. As a result of this, different parts of downtown have architecture from distinct historical periods.
In the 1700's, commercial activity was concentrated around Lower Water Street. This was "downtown Halifax" circa 1750-1840:
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By the 1850's, the nicest retail area was along Granville Street, two blocks farther up. These buildings are from that era:
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Then from around 1870-1900, the biggest retailers moved to Barrington Street, which ended up as a mix of mostly Victorian and early 20th century buildings:
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In 1960, Barrington was the busiest street and Gottingen was perhaps the second busiest area. By the 1990's, Spring Garden Road was the busiest retail area and Barrington was quite run down. Spring Garden Road evolved from a neighbourhood with a small main street in the 1970's into a state where it is just as built up as the older parts of downtown, but with a much higher population density. What happened to Barrington is similar to what happened to Yonge Street in Toronto, while the worst part of Gottingen reminds me of Hastings in Vancouver.
These days every part of inner city Halifax is gentrifying, and boundaries of the downtown area are getting fuzzier. I expect that some of the biggest developments will end up in outer areas of the urban core in the future because of height limits and limited available land in more central areas.