The letter to the Colossians is controversial.  Some claim its attacks on Gnosticism are anachronistic, since Gnosticism did not develop until the second century.  Others trace Gnostic development back two or three centuries earlier, which would make Paul's comments not out of place.

Some forms of Gnosticism posit a multiplicity of spheres of action, with the cosmos in a relatively outer sphere, controlled by a malificent archon.  The human soul, trapped inside the body, needs to learn secret passwords in order to escape to a better sphere.  The writer of Colossans actually uses some Gnostic terminology, such as "archons", "mystery", and "secrets", and warns his readers not to worship angels.  Oddly, there is also a warning against celebrating (Jewish?) festivals.

Did Paul ever visit Colossae?  The letter implies that he did not.  An oddity is that Colossae is not far from the main road between Antioch and Ephesus, which Paul traveled during his third missionary journey.  (Laodicaea, which he also seems not to have visited, is actually _on_ the road).  Possibly the letter was written before the third journey, in which case the best candidate for the place of writing would be Ephesus.