Breaching the Walls (TCG)

Note: This chapter refers to rules and concepts used in TSR'S BATTLESYSTEM™️ miniatures rules. Since siege warfare is, by its very nature, an activity involving scores, hundreds or even thousands of troops. BATTLESYSTEM™️ is the system of choice. Of course, this doesn't mean that small segments of a siege can't be roleplayed to great effect using standard AD&D® rules.
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==Breaching the Walls==
Sometimes an attacker has no options but to breach the castle's walls. In medieval times, this was attempted using a wide variety of siege engines-catapults, rams, and the like-which are discussed in more detail in a later section. Generally speaking, these siege engines had minimal effect on a well-built and stoutly-defended castle.
In the early Renaissance period, however, matters changed. Cannon appeared on the battlefield, and the days of the castle were numbered. Most AD&D game campaigns take place in worlds where gunpowder either doesn't explode or hasn't been discovered. The BATTLESYSTEM™️ rules bring in arquebusiers, however, and thus open the door for other firearms... including cannon, used by both defender and attacker. Details on siege guns are given in a later section.
The BATTLESYSTEM™️ rules (page 82) list the number of hits certain building features can take before being destroyed. While the information provided there is fine for most miniatures games, it must be expanded upon for use here.
The figures given for walls refer to sections 3" (30 scale) long. When the section is reduced to zero hits, a breach 1" wide appears in the middle of the 3" section. This isn't an instantaneous matter, of course, and if two breaches are created 1" or less apart, the wall between the breaches collapses as well.
When an object such as a gate or a wall is reduced to zero hits, it doesn't simply disappear: wreckage or rubble is inevitably left behind. In the case of doors or gates, the wreckage is considered to be an obstacle (i.e.. it costs 4" of movement to cross it). Troops receive a 1 modifier to their AR against missile fire that passes through such an obstacle.
When a wall is breached, it collapses into rubble. This pile of shattered rock is considered as rough/rocky terrain for movement and combat purposes. Troops "holding the breach" will almost always be at a higher elevation than troops attacking into the breach. A breach is nobody's "favored terrain".
Note: This method is quite different from the system described on page 76 of the AD&DR 2nd Edition Dungeon Master's Guide. While the latter method is useful for "quick and dirty" situations where the DM has to decide the results of a PC's or NPC's actions, the system described above and in the BATTLESYSTEM™️ rules is a more thorough and consistent treatment of the subject.
===Fantastic Creatures===
By their very mass, large creatures like dragons can be very effective against fortifications. Creatures of huge size have AD 10 against building features, while creatures of gargantuan size have AD 12.
Certain creatures have significant effects against fortifications for reasons other than sheer mass. For example, because of their kinship with the stone, earth elementals can do great damage. Treants, too, can tear down walls by insinuating their root-like limbs between the stones and tearing them apart. Details on this are given in a subsequent section.
===Magic===
In addition to useful spells like knock and passwall, there are several dweomers that do significant amounts of damage to fortifications. These are listed in the next section. Note that most of these spells don't appear in the BATTLESYSTEM™️ rules spell list. Players engaged in a BATTLESYSTEM™️ battle or campaign might agree to include them. using casting times and such as given in the AD&D game rules. If BATTLESYSTEM™️ rules are being used to decide a siege in an AD&D game, the DM should probably allow these spells to be used.