Gameplay
Terror takes place mostly underwater, with base-building and combat all being submerged beneath the waves. This is also used as a plot device; all of the alien technology from the first game is unusable in salt water, forcing the player to capture and develop new technology.
The game consists of two parts. The first is the real time-based GeoScape, a global view of Earth where the player views alien and X-COM craft and bases, can hire and dismiss staff, buy and sell vehicles, weaponry, ammunition and items, and build and expand bases.
The second part, the BattleScape, is used for combat between squads of aliens and humans, and takes the form of a turn-based battle from an isometric view. Underwater battles use the same physics as the ground ones.
Read more about this topic: X-COM: Terror From The Deep