Hero - 0 to 9,999 g.p.
Superhero - 10,000 to 19,999 g.p.
Lord (fights as Superhero +1) - 20,000 & up g.p.
Still playing Dungeon frequently. Above is a leveling system that we are now using. Each character begins the game as a Hero, and can progress to a Superhero and Lord based on treasure collected. Going up a level requires gathering the required treasure and returning to the start (Staircase) for "training". Treasures are retained after training, and if lost after this point your character stays at the level they have achieved. (A similar system could be used for Elf - Wizard, but we haven't tried it yet). If a character is killed you start again with a "fresh" character of the same level.
This allows for several victory conditions during the game:
-First to Superhero
-First to Lord
-First to find the Huge Diamond on Level 6.
-Most treasure when the game is ended
Here's another leveling system I spotted: Dungeon! Solitaire Variant by Delta
Here are some other alternate rules that we use in our games:
Each player rolls 2d6 for movement each turn. This gives an average move of 7 squares per turn, slightly faster than the 5 squares per turn in the rules.
Forming a Party
Two or more characters may join up to move and fight monsters together. After entering a room and drawing a monster card, the youngest player attacks first (alternately, highest roll on a single die goes first). If the monster is not defeated, each player rolls a single die, and low roll indicates the character attacked by the monster. If one character is seriously wounded or killed, they return to Start, and the other player must now fight the monster alone. First to slay the monster gets to keep the treasure. Characters can trade or give treasure to each other, such as an extra magic sword.
Edit: The above rule additions are for the 1981 version. The other versions (1975, 1989, 1992, 2012) may have slightly different rules. I believe the new version (2012) uses essentially the same rules as the 1981 version but renames the Hero as Dwarf Cleric and Superhero as Human Fighter.