There are several different kinds of Spiders present in Middle-Earth.
The AI of the spiders within the Lord of the Rings mod is very similar to that of the vanilla spider prior to 1.8, but the spiders of the LotR mod are much stronger, much faster, and significantly more dangerous; they're also aligned with various factions, so their hostility is affected not by light level, but by alignment. Various spiders can subject the player to various status effects, including poison, slowness, and frost.
Unlike in vanilla Minecraft, where only skeletons can ride a spider, creatures other than the undead are capable of riding certain types of Spiders. As in vanilla, the AI of the spider controls how both mobs move. The rider's AI plays a part too, however, as melee units will attack when the spider gets close enough, while ranged units will shoot at their enemies from a distance.
Evil players may be interested in hiring certain types of Spiders. They will be able to hire Mirkwood and Mordor spiders, along with spiders ridden by Orcs. Spiders can be a formidable unit in combat, as they can deal poison damage and climb right up over fortress walls. However, they are weaker than most units in one-on-one melee combat, due to their lack of armour, although their poison can do a very good job wearing down enemies, with the exception of Elves.
To defend a fortress against spiders, the simplest way is to make sure that there's an overhang around the edges.
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