Almost done with my very first FF1 playthrough, I think (just got to the mirage tower) and holy shit, I can already tell that Marsh Cave and Terra Cavern are going to be THAT part on future playthroughs.
>paralysis zombie ambushes
>poison spider ambushes
>cockatrice ambushes
>poison slimes
>massive wolf packs that waste your time
>piscodemon quartet guarding Astos' crown
>terra cavern has to be visited TWICE to actually beat it
>all this shit early in the game where money is tight and your mages need to conserve their limited spells