Thomas Shey
Legend
Good historical summary. I would only add that hexcrawl, leading to stronghold play, was a vital evolutionary axis which provided some flexibility in terms of what happened as PCs leveled in the original (1e and earlier/some of the Basic line) game, at least as-written. When your game began to sport PCs of level 7+ they would start moving out of the dungeon proper and move across the landscape, hexcrawling. This has a pretty structured process similar to dungeon crawling in 1e (it is just referred to the AH Survival game in OD&D, though there are encounter matrices available). Once these PCs get into stronghold/tower/whatever development then most of their 'calendar time' becomes absorbed and troupe play is supposed to refocus on their lower level henchmen/associates/alternate PCs, with the 'big boys' only reappearing in person for 'special events'.
Except, as I've noted before, there's little sign most people playing D&D were doing that even as early as 1975, whatever the theoretical intent.