Hi Sonya
Paul is addressing false teaching here. In the section before this i.e. Gal 2:11-16 he talks about having to reprimand Peter for teaching newly converted Gentiles to live like Jews.
He goes on to focus on the danger of this teaching. Nobody will be justified by the works of the law, but only in Christ (v16)
It is this [b]teaching[/b] (of being justified by the works of the law) that Paul is talking about having 'torn down'. He did this in his epistle to the Romans (see Romans 8v4).
EDIT - The argument of verses 17 and 18 is Man is guilty and needs a spotless righteousness. This righteousness can be obtained, not through law-keeping, but through Christ believing. A professor of the doctrine of salvation by faith who preaches salvation by works, re-erects the legal structure which he himself cast down and so proves himself to be a 'transgressor' in having thrown it down. But Christ told him to throw it down, and so Christ taught him to do what was sinful, therefore Christ is a minister of sin! "God forbid" exclaims the Apostle. The law is divinely perfect; man is hopelessly impotent, hence it is useless for him to seek righteousness by the Law. Christ Who is righteousness envisaged by the Law, becomes such to whosoever believes upon Him; and thus He glorifies the Law and redeems the sinner.
Hope this helps
David