To accomplish this task, either get on the airship as soon as your faction controls the Hangar and drop in from above, or use the Catapult to launch yourself in. Time is of the essence as at least one of the gates are usually destroyed fairly quickly.
To enter the opposing keep via the airship, your faction has to control the Hangar. Use the portal to teleport onto the airship. When the airship is close to the keep, leap off. The faster you are going, the further you will travel in your fall. That is, being mounted will increase your chances of landing within the walls of the keep. When you reach a little above the level of the walls, you will gain a parachute effect and fall gently to the ground.
To enter the keep via catapult, your faction has to control the Docks, where the Catapults spawn. Load yourself into a catapult ('mount' it as the driver), set the aiming point within the keep's courtyard, and launch yourself.
Patches and hotfixes
Patch 3.2.0 (04-Aug-2009): Added