Is there any way to tell whether a tower will be able to attack enemies at a particular point? I find myself confused by all the various stairs, ledges, etc, and while the area of effect overlay seems to ignore elevation, I'm not sure that is accurate. Is it just a matter of having line of sight, meaning I could "test" it by standing where I would place the tower and determining whether I can see the point in question? Or are there also limits on how far up or down towers can attack?
Two towers, Deadly strike and lightning towers can shoot through walls while others, like the fireball tower, magic missile, and squire harpoon cannot. The projectiles, when they hit a wall, will not continue through and thus cause no damage to the mobs.
Line of sight does play into the game for some of the maps, but all of the huntress traps, and monk auras have a spherical area of damage.
With towers like the Fireball, where it shows you an area of effect, there is a vertical limit to how far above and below the tower it can fire. It shows that in the area of effect, however, and it's pretty big. Place a tower on the roof on The Ramparts and move it around, paying attention to the ground next to the roof. As you move the tower up the roof, you will notice at some point that, even though the ground would still easily be within the area of effect, it is no longer green. (Note that you might need to have a decent number of points in tower range for this experiment to work.)
So there is a limit, but it's easily noticeable when placing a tower in game. Still, a Fireball tower can't shoot through walls or floors, so line of sight is important.