The Crystalline Towers are each visually unique, but functionally identical. The tower has to be built on a confluence of at least two ley lines, which is required to power them. The tower has a traditional stone footing built, and then the arch-mage or tower mage begins the arduous task of mapping and drawing out the hermetic seal required to draw power from the ley lines, and to summon both a crystalline earth elemental and a fire or storm elemental that are bound together to create the crystal that gives the tower their name. After the week long ritual is complete, the elementals are summoned, and the power of the seal fuses them together into a unique crystal upwards of twelve feet across. This crystal, its color and shape influenced by the types of elementals summoned, rises to approximately twenty to thirty feet in the air and begins to rotate slowly. Anything the crystal detects in it's immediate vicinity it responds to by hitting it with a powerful Burning Ray magical attack. The ray inflicts a substantial amount of damage, has armor penetrating capabilities, and discharges every other combat round.
In game use, the burning ray should be instantly fatal to low level threats, seriously dangerous to medium level threats, and high level threats can survive multiple hits, but the tower, if given enough time, can kill anything.
Combating a Tower
Facing a crystalline tower is difficult. The best way to deal with such a tower is to simply find out what its range is and skirt around it. The limited number of ley lines limits how many towers can be built, and where they can be built. The downside of this is that the people who build the towers also know all of this, and the easy paths around the crystal towers are likely to be patrolled or have conventional fortifications. Magic can be used to hide from the tower, but this is tricky, and if a concealment spell fails, it can leave the magic user hideously exposed to the full power of the burning ray.
Crystal shields exist that can deflect the burning ray, but there are several caveats to the crystal shield. The shields are fragile and if used in melee combat are very likely to shatter. They can also be shatter by being mishandled or dropped. If being used to block a magic attack, a critical roll from the attack will still be deflected by the shield, but the shield will be destroyed in the process. It will become blackened and cloudy, and will lose its magical aura. The final issue with the crystal shield, they are quite expensive to procure.
Siege Warfare is of questionable value against the tower. The burning ray isn't triggered by inanimate objects, so the tower wont target catapults, trebuchets, or battering rams, but it will target and kill anyone trying to use them. Siege golems are also of limited use, being animate and magical in nature, the tower will target them. A sustained effort can destroy a tower, and the crystal can be destroyed by a direct strike. The key is to provide the target with more targets than it can handle and overwhelm it.
If destroyed, the magic crystal powering the tower implodes back into the elemental planes, and the resulting counter explosion deals damage equivalent to that of a fireball spell cast at the level of the caster when the tower was made. Some of these magical blasts can be really impressive.
Being magical in nature, almost any magic user capable of building a crystalline tower would have the required skill to use the towers as a source of magical power, so that they can tap multiple towers for said power, so long as they were not active. The person controlling the tower would be aware of if it was active or not, and could use it for scrying and communication. Some more specialized magi could actually use a crystalline tower as a proxy for spellcasting, and cast magic spells through the tower. More than one group attempting to overwhelm a tower has been surprised by the appearance of magically summoned monsters, fireball strikes, or relocation spells.
Bypassing and Security
Depending on how a tower's seal was drawn, it could allow for the creation of amulets of protection that allow for safe passage. This is not common as an amulet can be lost, or stolen, and then the opposition has a way through a powerful barrier. There are also body seals, magic tattoos, that perform the same function, but a skilled artist and mage could counterfeit said seals and its back to square one. Most archmages will simply leave the towers on Kill, and what happens is what happens. A few will deactivate the tower in question when their allies are supposed to be passing by it.
Veavingax is most famous for her luminous burning ray towers, and these are the most common, but are by no means the only ones that exist. Magic is flexible and elemental affinities allow for a massive variety in towers that can be created.
Ice Towers are thankfully rare, but they are very easy to identify. Regardless of the weather, it is always cold near them, and once the edge of the Freezing Ray range is reached, everything is perpetually coated in some level of ice. In summer it is a steaming rime of ice, in winter, everything is burdened by the weight of ice. Those slain by the tower tend to last a long time, frozen, and only thawing to rot in the hottest months. Scavengers quickly learn to avoid these areas as the meat might seem ripe, but even they set off of the tower.
Storm Towers create a visual display, with random tendrils of lightning dancing between the crystal and its seal, and creeping out to touch things near the tower. When activated, the tower casts Lightning Bolt spells which are drawn to the heaviest armored foes first, but have a good chance to arc and hit several opponents at once. These towers are both bright, and exceedingly loud.
Tower of Fear - a much lesser version of the crystalline tower, a tower of fear doesn't generate any magical attack, just constantly radiates an aura of magical fear around it. This prevents the passage of domesticated animals and the weak willed. Heroes may pass, but their entourages, mounts, squires, pages, and valets are not going to.
Towers of Death are rarer still, and thankfully so. The tower has a Cause Serious Wounds attack that is area based, and can maim and wound large numbers of targets at the same time. Once every day, typically at midnight, the tower of death will release a dark pulse that acts as a Raise Dead spell, and any and all corpses in the vicinity rise as zombies slaved to the tower. Once risen, they will wander away from the tower in parabolic arcs that bring them back to the tower. Damaged zombies are restored by the dark pulse, and unless the skull is pulverized, the destroyed undead can be brought back again.