The good Snorri at the OD&D boards has posted an excellent rendition of the Outdoor Survival map that displays the whole thing (unlike Rob Conley's one IIRC) and saves you the trouble of going back to Vol. III and converting the symbols from '70s backpacking to their OD&D equivalents. The only thing missing are paths through the forests and mountains, like the one Potiphar followed on the initial trip to the Lost City, and fords in the river, like the one where you saw the battle between the bandits and the wereboars.
The Lost City is the crossed swords icon (e.g. lair) in the forest at the edge of the swamp in the middle of the western size of the board. Belltower is at the edge of the hex to the north of there, where the river enters the forest, but it's too small to be noted on the map.
The Patriarch Zekon's castle is in the hills to the southwest of the Lost City. You gather that the evil high priestess he mentioned is the castle to the northeast of Belltower, but you haven't been there yet. The lair symbol in the plains near there demarks the black dragon Scather's territory, even if no one you've talked to claims to know where his lair actually is.
Hruhrudingfallor is the city in the middle of the southern edge of the map. The civilized lands where some PCs have their backgrounds are further south, off that edge. I think that might be the FFC Blackmoor map if we ever need it.