The idea behind the modular spaceport is to have a very versatile structure designed to expand a moonbase layout and provide for integration of space ships and interesting activity for a static display.
The Spaceport is basically a series of basic moonbase corridors, to which tee junctions can be added. It starts with the straight that has the tower on it. From it, T-junction adaptors can be added to make the tower a tee or a cross. Alternately, a gate can be attached directly to the corridor without a tee. Gates can also be attached to the end of a corridor.
All you need to do is build more corridors, tee junction adaptors and gates and the Spaceport can be expanded to any size at all. For example, at Brickfest 2005, the Spaceport was 3×5 baseplates.
The images below show a couple of the many possible configurations of the Spaceport.