
Medellin is probably mostly know for the same named cocaine cartel run by Pablo Escobar, but that is now all history.
Medellin is not an extremely attractive town, more a industrial and commercial centre. At two million, it is Colombia's second city.
Most of the colonial architecture is long gone, but you might have a look at the churches in the centre. Basilica de la Candelaria seems to be the most popular.