Just my two cents, but I think it'd be simple and neat to be able to portal out at the end of each level, but should one choose to do so, they must fight their way back in, with no promise of loot or experience for levels already cleared. Perhaps the final floor contains a soul-recycler thingamajig that respawns enemys every time a level is exited? This could be any number of things: elder god, undead tyrant, ancient magic, whatever.
Being forced to go through all at once as many suggest would just be silly: it'd only be something you'd be able to do end-game, at which point the first levels would be mostly pointless, and in the end you'd have way to much loot, and way to little content left in the game to use it for.
Making it too easy though, as many have pointed out, takes much of the fun and challenge out of dungeon-delving.