The Obelisk is a mystical building that accumulates Energy over time.That Energy can be used in 2 manners: enable the defensive Shield around you city, or teleport the city to a new, random location.
Using any of those actions will consume 1000 units of Energy, which is also the level at which the Obelisk will no longer generate Energy.
The duration of the Shield can be increased by researching Divine Powers in the Protection Temple.
* Total Power amount provided on each level
Remember: the Obelisk does not generate any Energy when the Shield is up!