First of all, this will only work for those who play the alliance. You need to have The Hangar in order to make this work.
*Note: I haven't placed any image tags, because the pictures are way too big & I don't know how to resize them properly -.-' *
1. Get on the ship.
http://i27.tinypic.com/11t23rs.jpg
2. Wait till the ship is close to the horde's base.
http://i27.tinypic.com/255uy9v.jpg
3. Get up here.
http://i29.tinypic.com/zvsoch.jpg
4. Jump towards the thing I marked sticking out of the building. (It's solid, you should be able to stand on it). This may require some practice.
http://i29.tinypic.com/2cff85h.jpg
5. Walk trough the wall & tadaa! You're inside. ;D
http://i26.tinypic.com/qqwyli.jpg
6. Too bad the boss isn't here just yet :S (More proof that I am inside while the horde's walls are still up).
http://i27.tinypic.com/2ez48y0.jpg
My guess is that this is also possible using a catapult. But I haven't tried that yet. Enjoy it & make your allies go like ' WTFOMGROFLLLM0AA!!11!!111eleven!'.