Saint Louis was the capital of Senegal for about 300 years, until independence, when Dakar took over. Some 60 years prior, Dakar had already taking over the role of the capital of French west Africa, though for the last forty years or so before independence, Saint Louis did remain the capital of neighboring Mauritania. Indeed, for some four decades, Mauritania's capital was outside of it's borders, though only just.
Saint Louis grew on trade and, being administered by the French, thriving predominantly in the 19th century and because of it's history, became part of what is known as the 'Creole Atlantic', other examples of which are Havana and New Orleans. The city has been a UNESCO world heritage site since 2000. However, it will be hard for Saint Louis to beat the other competitors in its league.
As a tourist destination, the city hasn't too much too offer, though its hotels are good, if pricey, and it's restaurants quite decent, and some even excellent. But, apparently, it's the wildlife parks in the area which attract the fairest share of tourists, though in my opinion that constitutes rather little to warrant flying in from Europe for.