The Terraria Community Forums follow up on showing off Terraria: Otherworld at GDC with new details about the sandbox survival sequel (thanks VG247). The update includes their GDC trailer as well as details on the game's strategic side, saying this will include elements of the tower defense genre. Here's word:

Strategy Elements:

• The main objective of the game is not simply to explore, gather, build (although the player is still free to do this – we want to stress the fact that though this is a more structured approach, we are absolutely trying to retain as much player freedom as possible) - but rather to return the world to its pristine form.
• The player achieves this goal by pushing back the Corruption until it is no more. Your primary method towards achieving this goal will be finding/crafting and then activating Purifying Towers. These machines of ancient design will push back anything unnatural in the world.
• Not content to simply allow the player to purify the land, the vile force behind the world’s corruption fights back. The resonance from the crystals in the purifying tower attracts enemies, driving them to destroy the towers and re-corrupt the player’s hard-won terrain.
• The Purifying Towers need to be defended and even the best players would not be able to cover and defend the entire map at once (nor does that sound all that much fun)- so Terraria: Otherworld introduces another strategic element: Tower Defense. Through the use of customizable Wartowers – and any other clever defensive approaches - the player can ward off the advances of the evil forces without having to be physically present.
• These Wartowers can have a wide range of different weapons and defenses but are not self-powered. They need to be built in the vicinity of a Purifying Tower to get power from the Crystal.