A shipyard.

The Shipyard is the most important building to attack and defend. If it isn't built, you cannot attack any planet, and cannot defend unless an alliance helps. Many fleets (Also called ships) can be constructed manually when built. Building fleets may require certain technology from the Research Lab, and may also require upgrades of certain buildings. Also, upgrading the Shipyard can increase the speed of build time. The more upgrades the Shipyard has, the more faster you can build fleets.