Let's start here:
This is "Stonehenge" published by Graphic Products Corporation in their Formatt series of cut out lettering, used in paste-up before graphic design became something people just do on computers. Anyone have any sheets, let me know I'd like to get ahold of some.
Citadel aren't the only fantasy brand to use the font, it appears on TSR D&D products, Palladium Games RPG, Heritage Miniatures "Dungeon Dwellers" and many others. In GW publications, it's also used as the 1st and 2nd Edition Warhammer logos, and as each of the book headings in 2nd.
3rd edition logo is very heavily based on it, but it isn't the same thing really.
So far there have been two digital revivals of Stonehenge, both distributed for free:
1)
Moria Citadel by Russ Herschler of
Dragonfang - online gaming supplies retailer and archive of many an adventure sheet. Russ was kind enough to point me in the right direction for the origin of his font. This is the one I used on the logo up there at the top of the site, and so is an Offical Oldhammer™ Approved Product
2)
Satans Minions by Mickey Rossi, which isn't quite as nice IMHO.