How does Basic Fantasy RPG compare to actual old games instead of retroclones?
I haven’t played Basic Fantasy RPG or most other clones, since I still have my old books when I want an old school game. I have read BF and it is essentially a retroclone of Basic D&D {cleaned-up OD&D) with a few upgrades. You can download it free from Dragonsfoot, along with many campaign and adventure supplements, and supplements that add elements from AD&D.
The most noticeable changes I see are:
Race and class are separated as in AD&D, so you can have a dwarf thief, etc. Elves alone may take a combination class of fighter/magic-user or magic-user/thief.
AC #s are flipped to the modern progression, so better armor has a higher AC #, and a +1 armor piece makes numerical sense.
Attacks are resolved by adding an attack bonus to a d20 roll and comparing to the target AC #, exactly like 5e D&D, instead of using a table or THAC0 to resolve.
It looks like a solid option for anyone who wants to use a new supported product instead of the original old Basic/Expert books, especially if you want to use the modern AC/to hit mechanics. The expanded campaign topics like sea travel and hirelings are presented clearly and could be a good quick reference source to use with other OSR games.
The Haunted Keep appears in Moldvay's 1981 D&D Basic Rulebook as a partial sample dungeon. It's a good starting point for designing your own larger multi-level dungeon, but there are several finished versions out there that are ready to play.
DF23: The Haunted Keep is 35 page PDF available free on the Dragonsfoot publications page. I might run it after my current campaign finishes B1 and B2.
(Dragonsfoot bills itself as "the home of 1st Edition AD&D" but there is also a wealth of material there for OD&D, 2nd Ed, or any OSR clone you want to play. If you haven't been there before, be prepared to lose a lot of time when you click the link.)
A different adventure with the same inspiration that I haven't read yet is Shadow of the Haunted Keep, available from DriveThru RPG/RPGnow.