The Titans were enormous mechanisms built in the past, modeled upon the Giants. There's a total of 4 Titans shown in the game: one in each direction.
One Titan was dismantled by the Lizard People of the South, where its heart and eyes lie fully functional in the labs beneath the Barren Hills for reasons unknown. In speculation it presumably represents the production of creatures used for fighting, and a potential revival.
A mysterious skeleton lies in the beginning of the West Zone. We don't know for sure that it was indeed a Titan, as this skeleton is white, whereas the other Titans are purple.
Lore and Story Analysis
The Titans were built - for unknown reasons - by an ancient race in the distant past, the Librarians. The Titans were modeled upon giant creatures that walked the World at this time. It is very likely that these creatures were a mesh of flesh and (or just) machine.
- Theories about the Titans, their purpose and the consequences. Perhaps...
- ...Titans were created to fight the Giants.
- ...the Giants had their own extinction war with the Titans.
- ...like nuclear weapons today, they were simply a deterrent.
- ...the Titans were simply massive worker machines, or trophies. A symbol of technological achievement.
- ...their original purpose was not for war.
- ...is the Pink Energy source itself that corrupts.
- ...they led to the disappearance of this ancient race.
As a matter of fact. it is never mentioned, how large the titans are. But at the beginning of the game, three titans are seen emerging from the clouds. That taken into count, it is quite possible that the titans were from 1-2 kilometers tall as the first clouds only appear after 1.5 kilometers from the ground level.