The Micronauts: Micropolis

This is the Micronauts “building set that never stops growing” from Mego  

As the box art suggests: “Make this and dozens of fantastic space age structures for your micronauts.” 

These space-age construction sets “enable your child to build any number of different toys that can be taken apart and put together over and over again.  Using a simple five millimeter plug and receptacle system, Micropolis building sets will adapt to all Micropolis figures and accessories."

I have two Micropolis sets in my collection, the Interplanetary Headquarters and Mega City (which consists of 579 parts).  

Joel and I have a lot of fun building new configurations together, but the "plug and receptacle system" can really, really hurt your fingers...