The standard for the gameport is 2 joysticks with a total of 4 buttons, but if you want to be a little creative you can actually get a config of 1 joystick and EIGHT buttons!
How? A joystick has 4 microswitches, so why not simply wire up 4 buttons to the same inputs as for the 2nd joystick instead??
If you are using an old version of MAME then this will work a treat. The code was changed, I think around v0.53, and support for Dual Joysticks was omitted.
Email me direct if you want any more info on this - we have done quite a few custom solutions via directly wiring to a joystick extension cable and it always works out well.
So to those of you who say you cannot have a Defender layout that connects entirely to the Gameport, I challenge you to simply think outside the square.......
You don't really need perforated board either- all you need is a few wires, 4 resistors, a soldering iron and the ability to read a simple circuit diagram.