[quote=akje]
Gates, when were there gates? I figured it was like a ship. Open on top. As for the floaty bit, Martin jumped. You posted 10 meters high. So he climbed the church tower and jumped the last bit.
[/quote]

I specifically posted it was a floating bunker. There are no openings. You could potentially sneak in through the openings on front, where the Missile Launchers are currently deployed, but that place is crawling with people, and all activity there is monitored. You'd have a hell of a time breaking through. And the only thing that could potentially make Terebithia land would be a message from SOLDIER.