The Fortaleza de São José is Macapá's best sight: a broad, low, stone-and-earthwork fort begun in the 1760s and inaugurated in 1782 to defend the Portuguese Amazon from rival European powers. It is more impressive for its scale than for ornament, with thick ramparts, angular bastions, and views across the river.
The fort sits beside the riverfront park, so it works well with a walk along the Amazon. Go early or late in the day; the open stonework can be brutally hot at noon.