More or less the definition of timelessness in gaming.
The overworld is actually large enough to get confusing--the original cartridge comes w/ a map to help you out--and there are nine separate dungeon levels, called labyrinths. These can be entered and played in any order though some areas of the map and certain labyrinths require items in the previous labyrinths to get to. There is nothing to stop you from entering one dungeon, retrieving the needed item and leaving. It's nice if you get frustrated on one area and need to take a break from it. Mostly, you will want to beat the labyrinths in order though as they scale in difficulty and you receive an additional heart in your life meeter after you beat any labyrinth's boss.
These additional hearts--called heat containers in game--along w/ collected items provide for a little bit of an RPG-style leveling up. The extra heart containers and other life-saving power ups are absolutely necessary as the difficulty scales because some monsters in later levels are quite powerful. The majority of your improvement though comes from practicing and acquiring skill. Many areas of the labyrinth play almost like an old school avoider/shooter like Asteroids. It's fun but it can be a challenge that's not for the faint of heart. No matter if you collect ever single power-up or not, expect to either die quite a few times or make liberal use of your reset button.
The biggest problem--and one that continues to be a problem in this series to this day--is that there are occasionally secrets and items that must be found for which there are no in-game hints as far as their locations or even, in some cases, that they are even necessary. First time players who are averse to using strategy guides and FAQs should be prepared for a lot of frustrating and seemingly endless wandering. Beware though that much of the challenge is in finding the various labyrinths and secrets so relying too heavily on walkthroughs and the like takes a lot of the fun out of it.
W/ a little help, most people can probably finish the game in a day or so of gaming. (If you insist on not looking up any secrets it could go on for God knows how long.) There is an additional, tougher quest to be played after the first one but most of the replayability of this comes either from nostalgia or coming up w/ additional challenges of your own. It's simple and sweet but exceptionally enduring in its old-school charm. I will never be able to see this w/ unbiased, fresh eyes but as someone who has been playing this from a young age, it holds up now as well as it ever did. If you're thinking of replaying this, do it. If you've never played before, good luck!