1. Both of them take place in an underground land, where no sunlight can reach.
Seriously, what are the options? Either you are on a planetary surface, or you are underground. And if you compare the number of settings that take place on a planetary surface to the number of underground settings, I think underground setting comes up pretty original. In fact, the fact that people actually
do inquire about the originality of this proves that is
is pretty original, because noone would think twice about surface settings.
2. They both use a great crystal to light the land, and smaller crystals can be used to light rooms, buildings, etc.
(in the seventh tower, they are called sunstones)
Again, what are the choices, usually? It is almost a natural choice to conduct light from the surface down through a crystal, just as in fiber optics. An alternative of using magic would result in difficulties in explaining plant growth, and a very high percentage of completely deserted spaces.
Likewise, any other, conventional artificial lighting like lamps produce the same effects, and add the problem of oxygen.
Another solution would be light emitting plants. However, that presents the problem of how they actually managed tro evolve, how they manage to produce sufficient light and
why they would do that, in terms of evolutionary reason. Emitting light uses quite some energy, so doing so needs to be carefully weighed in a cost/benefit comparison if a species is to survive.
Additionally, PS does
not use smaller crystals to light rooms. It uses normal medieval-style lamps, candles, etc., as can easily be seen not only ingame, but also on the mainsite's flash animation of a tavern scene.
3. Planeshift has 7 levels in its stelactite, and the seventh tower's land has 7 levels to it also
Planeshift has not 7, but
8 levels. Two of them are completely submerged at all times, whereas one is submerged depending on season. I also, while not having read the link you gave yet, think it's pretty safe to assume that TST doesn't have the same or a comparable race set, with the exception of the obvious and standard fantasy races (humans, dwarves, elves).
Thus the conclusion is that what is similar are things that follow from the base considerations, and thusly are very easy to make up independantly, so I vote "Coincidence".
