From publisher blurb:
The BX Advanced Gold Edition rules are based on the 1978 advanced first edition edition of the world’s most popular fantasy roleplaying game with the less complex game systems of the 1981 B/X edition. In other words, this is a modern version of the fantasy roleplaying game many people played in the 1980s. They started playing with the B/X edition and just added the races, classes, spells, monsters and treasures from the first Advanced edition, instead of using all the more complex rules from the Advanced Edition. All of the first advanced edition fun without all the often complex and fiddly rules.
In BX Advanced, you may play a first edition style assassin, barbarian, bard, cleric, druid, fighter, illusionist, magic-User, monk, paladin, ranger, or thief in an old school game of exploration, treasure-finding, and fast to play combat. While B/X systems are used instead of the more complex first advanced edition rules, many of the limits of B/X have been removed: there are 20+ levels instead of 14; race is separate from class, spells go to 9th level instead of 6th, and there are many more monsters and treasure items than in B/X. Be a hero or a rogue: your decisions determine whether your character will retire rich, rule a kingdom, or die before his time.
This modern version also includes many optional rules: psionics, some of the house rules the author used in the 1970s, and selected ideas from other roleplaying games. While a sample adventure and campaign setting are included, these rules can easily be used with all of the old adventures designed for B/X or 1e and the many new adventures published for these games in the last few years – or you can easily create adventures of your own.