Sky Passages can be upgraded with Safe Passage, which reduces the cost and adds a weak attack to defend themselves. Because up to 10 Sky Passages can be built, and because they allow for an unlimited number of teleports, Sky Passages are one of the most tactically sound buildings in the game.
In-game information Edit
"The Sky Passage allows armies or Citizens, through the power of the Titan Oranos, to travel instantly across vast distances. To use the power of the Sky Passage, select a unit or group of units and select the garrison button on the lower part of the screen (it looks like an opening door with an arrow pointing through it). Your units will go inside the Passage and be able to step out of any other Sky Passage (select another Sky Passage and click the garrison button again to release your units). Remember that you must have at least two Sky Passages to use this ability. If you have more than two Sky Passages your units may step out of any one of them."