MONSTERS AND ORGANIZATION
This section illustrates a part of the art of DMing that is perhaps lost on a lot of people now, but which post-3.x D&D actually tried to bring back a bit (to its credit).
Monsters use tactics and behave in logical fashion.
The section deals heavily with the idea that DM must be prepared for all eventualities, and should prepare in advance the course of action a given monster or group of monsters will take in the event PCs take any number of logical possible approaches to them. While a DM must be able to think on his feet and it's true PCs will often think of things the DM did not, this section isn't concerned with detail-oriented breakdowns of every PC strategy. Rather, the idea is, "how will monsters react if surprised/ambushed, if openly engaged, or if an effort to parlay is made." These are the three basic things that can happen in an encounter.
Also of importance here is what will happen if the PCs leave and return to encounter the creatures a second time--some creatures will flee; others will call for reinforcements; still others will set canny traps and build defenses. All of this may seem like common sense, but having notes, in my experience, is always helpful. Indeed, Gygax even points out that sometimes players throw tantrums over the results of a failed effort, and will even accuse the GM of acting with foreknowledge, or metagaming the monsters--having written notes about monster reactions beforehand allows you to show the players the notes and shoot down any such accusations.
Does this happen? Yes. yes, it indeed does--I have seen it happen, and I have had it happen to me.
Having notes and engaging in preparation never hurts.
The section is capped off by six wonderful examples of initial encounter/return engagement scenarios from monsters ranging from zombies to giant ants to orcs to bandits to a fortress. Very, very useful information herein.
USE OF NON-HUMAN TROOPS
This is a pretty neat section--it discusses not only the idea that humanoid troops (orcs, goblins, bugbears and the like) are not only difficult to keep in line due to their barbaric, savage nature, but goes into such detail as what exactly their loyalty base will be if they have a strong leader, officers, and how they will react to members of other species. There is a table identical to that in the Players Handbook, which focuses on humanoid racial preferences for compatibility. I found it amusing that Hobgoblins, Orcs, and Trolls have no Preferred race--Orcs even regard other orcs with open hostility (assuming said other orcs are of a different tribe). The best it gets with Orcs is that they tolerate Goblins...unfortunate, since Goblins have a generally negative (though neutral overall) outlook on Orcs.
The only way, the section explains, to control non-human (humanoid) troops is to give them "cause to fear disobedience." (p. 105) Strong leaders are essential to controlling populations of humanoid troops within an army, and the text mentions that such troops will "fight with humans nearby to (sic) whether friendly or not...run from battle if they see troops on their own side retiring or retreating, and fall to looting at first opportunity." (p. 105)
I suppose one has to assume there's a good reason why orcs, goblins, lizard men, and the like haven't overrun the world and are secondary to the human and demi-human races; "they're evil" wouldn't necessarily cut it. Sheer barbarism like that described above, on the other hand...
CONSTRUCTION & SIEGE
I'm not going to spend a great deal of time on this section, as
there's not a whole lot to pick apart. I could go into great lengths
about the actual costs and time in constructing a castle vs. those
listed in the book, but that's not really important--especially
considering that we are looking at AD&D economy here, not real-world
medieval costs. However, it is one of those hidden gems in the DMG that too many folks forget about, and it's definitely worth revisiting even if you're an experienced DM.
Underground Construction, Construction Time, Constructions
Suffice it to say, these are pretty cool section that answers quite well the question of, "so I've reached name level and get to build a fortification. What now?" There are very detailed breakdowns here of the costs to create everything from a 10' section of wall to an arrow slit, and how much time it takes to dig underground tunnels and chambers based on the race of the diggers; even how many dwarves can fit into a 10' wide shaft is explored here. There are dimensions and descriptions given of everything from the arrow slit (1/2' w, 4' l) to a stone gatehouse, to a murder hole.
Siege Engines and Devices of War, War Machine Fire Tables, Siege Attack Values
Ever wonder where the rules are in AD&D for running a siege? Here they are. While the book is quite clear that mass combat rules are beyond its scope (they came earlier in Swords & Spells and much later in the Battlesystem rules, and more recently have been covered by Delta in his well-written Book of War supplement), it does provide a system for the use of siege weapons against structures and individual PCs. The system is pretty straightforward and directs one towards the Grenade-Like Missile rules (pp. 64-65) to determine where a launched projectile from a siege engine lands if it misses. Statistics for a variety of siege weapons are given, along with their cost in gold pieces, range, rate of fire, minimum and maximum crew to use (and the effects if more or less than the minimum are available), as well as base AC numbers to use for targets and d20 roll modifiers based on such things as target movement, size, and weather conditions.
One of the more interesting inclusions here is the "Siege Attack Values" table--this is because it gives a breakdown of how much damage certain spells deal to structures. This means if you've ever wondered how much damage casting fireball at a castle wall deals to the wall...it's here.
Talking of damage, this section also gives us the hit points (called "Defensive Point Values") of each of the aforementioned construction items--a curtain wall section 10x10x10 (thickness, width, height), for example, has 20 DPV (hit points). The difference in terminology, presumable, is to hammer home the idea that your broadsword isn't going to cut through a castle wall, regardless of your ability to deal 30 points a round with it.
This section of the DMG is impressively detailed given how small a space it takes up (only about 2 pages total), and would be invaluable for any DM who is going to run a siege or mass battle--these are pages that you should flag with a sticky-note or bookmark.