Challenging the Characters

Balanced Combats

The most obvious method of challenging the player characters' abilities is to test their skill in battle. Combat is a big part of the AD&D game, and properly balancing the combat is one of the DM's most important responsibilities. Unfortunately, the only way to design a completely balanced encounter is to apply hard-won experience. Until you have this experience, try to err on the side of making the foes too easy for the characters to vanquish. It's easier to add more enemies (or beef up existing enemies) during play than it is to mysteriously remove enemies from the battlefield.
Typical adventures make use of three different types of combat encounters: romps, battles of attrition, and drag-out fights. Romps are combats in which the player characters aren't in any danger; they're designed to allow the characters to kick some butt and show off. A good example might be a small party of goblin raiders trying to waylay a party of 6th-level adventurers. Although your ultimate intention is to push their characters' abilities to the limit, allowing them to run amok tends to increase the players' sense of satisfaction and create some good opportunities for roleplaying. Although this formula isn't completely reliable, most encounters in which the total Hit Dice of the opposition is equal to or less than one-fourth the total experience levels of all the adventurers are bound to be romps. In other words, eight orcs (1 Hit Die each, total of 8 Hit Dice) versus four 8th-level characters (total of 32 levels) is a romp.
Battles of attrition are engagements that pit the characters against slightly tougher foes. Although the characters should win these fights, the opposition might be stern enough to inflict a little damage. The idea here is to test the party's stamina. Alone, none of the battles presents a significant challenge to the heroes' abilities, but a series of these combats should start to inflict a real toll. Do the PCs have enough hit points to withstand the onslaught? Enough healing spells? Encounters in which the total Hit Dice of the opposition is roughly half the total experience levels of the adventurers often fit best into this category.
Drag-out fights are big, climactic battles in which the players face strong opposition. Normally, a drag-out fight is a prelude to the players' receiving some great reward-obtaining valuable treasure, uncovering an important secret, or successfully completing the adventure. Drag-out fights are the hardest combats to balance. The ultimate objective is to make the opposition weak enough that the players will almost certainly win the fight, but strong enough to make the players doubt their chances. As a rule of thumb, when designing your drag-out fights, begin by selecting a group of monsters with total Hit Dice that equal approximately 75% of the total experience levels of all adventurers. Since you're facing a much thinner margin of error when designing these encounters, though, you should be especially careful. Although the "75% Rule" might provide you with a decent starting point when setting up your drag-out fights, in the end you're going to have to trust your judgment. For now, just choose opponents that make the battle seem balanced in your eyes, and then take away one or two enemies or capabilities for good measure.
One of the keys to designing an entertaining adventure is to combine romps, battles of attrition, and drag-out fights in just the right proportion. The best adventurers establish a sort of combat "rhythm." For example, one or two romps can whet the players' appetites. These are followed by a few battles of attrition that lead to the first drag-out fight, which is then followed by a few more romps and then another drag-out fight. From here, the adventurers might face a series of battles of attrition that culminate in the big drag-out fight that serves as the adventure's climax. In effect, you're pacing your adventure in much the same way a good director paces a movie. Place romps in the areas where the surroundings are less interesting and you'd like to speed up the action. Battles of attrition are a good way to provide the players with the feeling that they are slowly but steadily advancing toward some goal. Drag-out fights pick up the tension and get the players' hearts beating a bit faster. Although it's dangerous to make firm assumptions about the order in which the players will tackle the encounters you sprinkle in their path, you should give some thought to how you'd like your adventure to progress when you are designing the players' opposition.

Heroic Feats and Tests

The AD&D game provides several different systems that allow the players to use their characters' unique abilities to perform heroic stunts and feats: ability checks, saving throws, and proficiency checks. These systems can be used to force the players to overcome a wide variety of obstacles that aren't directly connected to combat.
For example, you might require the adventurers to leap across a narrow chasm (requiring a Dexterity check). Similarly, you might place an extremely valuable jewel in a room full of worthless trinkets; only a successful Appraising proficiency check allows the adventurers to make off with the real prize. For more inspiration, you should re-read the sections on ability checks, proficiency checks, and saving throws in both the Player's Handbook and the Dungeon Master Guide.
One important distinction to keep in mind as you sprinkle opportunities to perform heroic feats and tests throughout your adventure is the difference between ability checks and saving throws. Saving throws improve dramatically with experience level, while ability checks do not. Since a character's experience level is the most important indicator of that character's significance and general aptitude, try to arrange things so that the adventure's most critical heroic feats are resolved by saving throws. As an example, suppose you've decided that, to reach the leader of the evil cult and rescue the princess, the heroes must first make their way through the Caverns of Despair-a magical labyrinth that saps their will to fight. Only heroes with the strongest willpower can hope to negotiate the caverns successfully and reach their quarry. Since Wisdom is a measure of personal willpower, it might seem like a good idea to require each adventurer to make a successful Wisdom check to make his or her way through the caverns. However, this situation is probably best handled with a saving throw vs. paralyzation, since it should always be easier for higher level characters to perform a feat that is so important to the adventure. There's certainly something wrong if a legendary hero has no better chance to complete an important test than a 1st-level character. In this particular case, you can account for characters with high Wisdom scores by allowing them to add their Magical Defense Adjustment to their saving throws.
For similar reasons, an ability check or proficiency check should never be used to resolve a life-and-death situation; always use saving throws to handle these cases. Let's return to the example of the narrow chasm the adventurers must leap to reach a remote portion of the dungeon. While it's reasonable to require a Dexterity check to perform the leap, any characters who fail the check should not fall to their deaths. Instead, give these unfortunates a saving throw vs. breath weapon to catch themselves on a ledge before plummeting. Again, the higher the hero's level, the greater the chance he or she will find a way to survive against all odds. Using saving throws in this fashion also ties nicely to some of the game's other systems and assumptions. After all, shouldn't a ring of protection +2 help prevent an adventurer from stumbling down a chasm to his or her death? It will if you resolve the situation with a saving throw, but it won't if you rely solely upon an ability check.