It is easy to make quests harder. Do any of the following:
-Just take away equipment (robbed in the middle of the night) half of all artefacts are missing and every other weapon costing over 400 gold is gone.
-illusion tricks you into attacking stone... roll one red die to see if weapon breaks.
-give monsters with RUST spells to counter wealth (too much gold).
-Use an invincible monster such as a cursed spirit that reappears where it was defeated one turn later.
-The emperor requests that you return several artefacts to his vaults...
-Plague quest, causes heroes to deplete their supply of healing spells and potions (they need to heal others in the dungeon / trade them for information)
-Kill all of the heroes. Once they have no equipment, the game can be impossible!
Really the biggest factor in quests being easy is the carry-over of healing potions from quest to quest. You could simply take some / all away stating that there were overland encounters causing them to be used.
The other factor is the lame "lining up in the doorways and waiting for the monsters." This is effectively countered by monsters with range weapons (even picking up rocks), monsters that open doors, two doors to the same room, and time based events... take to long to clear a room and monsters appear from behind and attack.
Other than adults over-thinking the game play with a predictable crawl mechanic, the game IS for adults. So I really don't get the question. Frankly I think "levelling up" is an out for lack of imagination. The unaltered game is about as close to perfect as you can get.