It doesn't need to be explicitly stated so the simpler minds can grasp that you're defending a beacon for x amount of time or waves of enemies, as shown in footage and news about the game. This is barebones tower defense genre.
The genre of a game is not set by its mechanics, but by the reasons one plays the game for. Terraria Otherworld might or not have one or more elements drawn from tower defense games, but that's not what the game is about, nor what it revolves around of. Terraria Otherworld remains a sandbox RPG because of the sense of adventure, exploration, progression and heroism it will give to the player. So has Terraria.
Dismissing Portal as a
ty first person shooter is ridiculous, even though one uses a gun, and plays in first person. Why? Because it is a
puzzle game and an adventure game; only a
few of its elements and mechanics resemble FPSs, and are simply used as tools to deliver the intended feeling and true purpose of the game. Games are flexible. Though I hate first person shooter games, Portal was amazing, and I wouldn't mind rating it as one of the most genius games ever created (along with its sequel, of course).
There might be turrets in Terraria Otherworld, but it would be
ignorant to instantly classify the game as one genre. Even worse is classifying it as one of the
bad examples of that genre: not all tower defense games are terrible children games where you stand still and do nothing.
In fact, standing still and doing nothing is not part of a tower defense game; it is merely an element most commonly found in those types of games. They can be oh so much more than that.
This is why I find your comments insulting; you seem to just want to throw Terraria Otherworld in a pile of games that you loathe because of one element present in it. You don't want to believe that a game can be much more than what it seems and than what a few of its mechanics are. You haven't watched real gameplay of a game yet insist, in advance, that it will be horrible.
Please try and understand how games work; why you play them, and why you love or dislike them. You might be impressed.