There are thousands of custom maps made for Creeper World 1. All of them can be found here:
Creeper World 1 Custom Maps
To play custom maps you need a full version of the game with the latest patch 0632

And the editor is here:
Creeper World Map Editor

On the forums a several threads started about custom map-making some notable ones are:

Graphics Tutorial by Sauffaus about creating custom backgrounds for the maps.

How to edit Creeper World custom map (.cwm) files by hand about editing the .cwm map files using a different editor allowing some interesting options unavailable in the regular map editor.