Most of the time ZZT RPGs are nothing like 'proper' RPG games; they may have the usual fantasy settings, the odd turn- based battle and perhaps a castle or two, but that's usually where the comparisons end. In The Mercenary, however, WCB (for it is he) has managed to include the kind of complex RPG bells and whistles in The Mercenary that most people would think impossible to create in ZZT.
The gameplay is the usual dungeon-crawler fare - slay monsters, get gems etc. - but WCB has made things a little more complex by including a 'level' system. Each time the you kill a monster, you'll get a little more experience. Getting enough experience allows the player to move up a level, just like in traditional RPGs. The higher the level, the stronger the player's character becomes. This allows you to carry more ammo and torches, and increase your maximum health. Further innovation comes from the choice of a "relic" at the start of the game. This is a special item that gives the player certain abilities, thus changing the course of the game. The lockpick allows you to enter, um, locked doors, the Gauntlet provides superhuman strength so that you can smash boulders, and the rope allows access to otherwise unattainable areas. Depending on which you pick, the game will proceed in a different direction, thereby adding to the replay value that I hear is so popular with the kids these days.
Structurally, it's very similar to Diablo - the player begins in a small town, before proceeding to kill monsters in a series of nearby caves an catacombs, eventually destroying the evil at the heart of the dungeon. This, however, is its biggest flaw; Whilst Diablo's simplicity and lack of plot was made compelling by the various weapons and armour that could be found, The Mercenary has no such options, so the player has no real drive to complete the game, except to build up the statistics. That's not to say you won't find it fun to play, just that the lack of a plot sometimes drags a little.
Aside from this, The Mercenary is a well-crafted, excellently programmed and cleverly designed game and certainly worthy of this month's GOTM.