HM's Castle of ZZT, not to be confused with David Newton's Castle of ZZT, puts you in, well, a castle, with the excuse plot of saving a princess trapped in a tower by the creations of an ill-fated mad scientist. It begins with a parody of the archetypal prison escape that goes by refreshingly (and, of course, humorously) quickly. Then you are faced with an invisible wall maze, a conveyor room based on the Mixer from Town of ZZT, a mad dash through a horseshoe path lined with guns, a traipse around shark-infested lakes, and a fight with a horde of tigers before ending with an RPG battle against three dragons.
What's notable is that all of these action-packed challenges, other than the RPG battle, fit in a single board. Thus, they are short enough that none of them drag out, and they are small enough to make their hazards more effective (e.g. the sharks are actually dangerous here, unlike in other worlds) or less frustrating (the invisible wall maze, done with a mix of normals and fakes instead of invisible tiles, would be horrid in any size besides the tiny one it is).
What they are not is forgiving: you have just one chance to heal, a cache of 16 gems that only gives you one extra hit point (the other four gems on the board are sealed off by an invisible wall that can't be removed or circumvented without cheating as a self-admitted "sick joke"). Combined with the gun gauntlet, you can easily end up with single-digit health before the tiger fight if you're not careful enough. Still, though, Castle of ZZT is remarkable in how much gameplay it's able to fit into a single board, in a difficult but fair fashion.
The centerpiece at the time of release, however, was the RPG battle, whose engine would later appear alongside the King's Quest ZZT battle engine in the ZZT Encyclopedia starting with 3C. Opting not to try to replicate the complexity of a computer or console RPG battle, it gives you just three options against your opponents: "shoot" for low damage at the cost of one bullet, "bomb" for high damage up to five times (assuming you found the grenades beforehand), and the humorous choice to "surrender" for a game over if you're somehow completely out of bombs and ammunition.
While it likely stood out at the time for its elaborate attack animations, this engine also stands head and shoulders above its contemporaries and later engines in many ways besides visuals. Unlike later worlds, it mercifully does not try to recreate the "Active Time Battle" system found in the Final Fantasy series, and thus is completely turn-based. This eliminates the need to rush your decisions before the enemy acts, or wait for a meter to fill to take your turn while you're getting pummeled. It is certainly far less frenetic than the King's Quest ZZT engine, which necessitates hammering attack buttons in a frantic effort to interrupt a more powerful enemy's attempts to attack so they can't erase your life bar in a flash. In fact, none of your three foes can deal more than two hit points of damage, whereas your gun does two points of damage and your bomb does eight, so the engine distinctly favors you in that department.
The engine scores additional points for being more honest than later engines: Castle of ZZT is up front about its simplicity and sticks to three distinct choices presented directly on the board, whereas other engines that use dialog box menus seem to love pretending that your magic spells are more than just different amounts of damage.
Lastly, it is vastly more polished than other engines of its vintage. No bugs are present affecting the flow of combat, and try as I might, I could not break the engine by e.g. trying to perform an attack while another is in progress or throw more bombs than you can carry. Its life bars do not use breakables shot with bullets, but instead use an object that quickly overwrites the life bar tiles with empties while checking on every step if it's reached the end. Thus it is not possible for an animation to be interrupted by a slow-moving ZZT bullet hitting the "kill" object, causing all sorts of logistical errors.
It is, however, a rather hard fight. Like other RPG battle engines, whether an attack hits is chosen pseudo-randomly. However, the randomness is tilted in your favor, giving you a 3:4 chance of hitting (and the bomb getting a second 1:2 chance of hitting if the initial check fails), while enemy attacks only get a 1:2 chance. Alas, the fight is specifically designed to be a "close" one even if you win, so getting particularly unlucky rolls of the dice, like missing attacks or taking two points of damage multiple times in a row, will lead to your swift death. Thus, like in other engines, you may need to reload a save many times, or even save multiple times mid-fight, till the fight turns in your favor. The fact that this is the last challenge before the game ends makes this more tolerable than in other worlds with many such battles, placing it on the level of the obstacles faced on the main board even if it's eclipsed by RPG battle engines created in the modern era.
Ultimately, Castle of ZZT is a tough but tight action experience that is worth your time. Try this out as soon as you can.