In the 18th century, the guitar was a chamber instrument, meaning that it was played mostly in the home in small gatherings of friends and/or family, therefore very little "concert hall" music was composed for the instrument. What pieces we have are mostly from the early 18th century Baroque period, as it was an extremely popular chamber instrument all over Europe.
The following is a bullet list of the guitar's progression. (Thanks to this website: Evolution of the 19th Century Guitar.)
-The 6-course guitar arose first in Spain in the 1750's, with double strings (same as today's 12-string guitar)
-Merits of single vs. double stringing was debated on 5 and 6 course guitars since at least the 1770's
-String improvements allowed cheap and readily accessible wire-wound basses in the 1780's
-Wire-wound strings cut into gut frets and necessitated metal frets
-Wire-wound bass strings were overpowering with double courses and required single courses for balance
-New styles of playing in the late 18th century necessitated a strong bass and clean articulation
-String lengths on Baroque and early Spanish 6-string instruments were longer than a concert Ramirez
-Treble clef notation replaced tablature in the 1760's
-Guitar pitch was raised to standard orchestral pitch with the adoption of treble clef notation
-The French Lyre guitar was a critical step toward the adoption of the 6-string guitar
-Single stringing was done initially by leaving half the string slots empty
-6-string guitars were around since the mid 1770's, but were not popular until the late 1790's
-The 18th century was not a period of musical decline. It was extremely active.
-The 5-course guitar remained popular in France until the 1820's and co-existed with the 6-string guitar
-The 12-string 6 double-course guitar remained popular in Spain until the 1830's and co-existed with the 6-string guitar
-The English and Germans played a form of cittern in the late 18th century and not the guitar
-The Italian guitar was single-strung, with 5-7 strings
The following video features Baroque guitarist, Paul O'Dette performing works by Santiago de Murcia (1685-1732) on baroque guitar at the New York Guitar Festival's third biennial Guitar Marathon at the 92nd Street Y's Kaufman Auditorium.