The worldbuilder itself is remarkably easy to use, since it can generate a new map in a matter of minutes. While you can painstakingly build a giant map tile by tile, you can also use the random map generator to create one of the game's basic map types (Pangaea, archipelago, and so on) and use a paintbrush tool to change any and all of the terrain on the map (so you can add a nice schmear of forest through your pregenerated desert with a single stroke of the mouse). The worldbuilder also has numerous "ploppers" that let you drop down resources, starting cities, city-states, initial units, ruins, barbarians, and whichever other features you prefer, with a single click. You can even adjust the starting fog-of-war level to clear however much, or little, fog from the map's starting state you like, essentially to create a scenario where the "known world" extends only so far.

And you can easily tweak your own custom maps after saving what you're working on and then exiting the tools, firing up the game, and loading up the map to play it immediately. You can also use the game's "tuner" tool in-game, which lets you change in-game values and units in real time over the course of an actual game (making it a powerful cheating tool as well…but who'd ever cheat in a game of Civ, right?). Of course, if you prefer to go more in-depth, you can dig into the game's mod tools, which are, like Civ IV's, based on LUA and XML scripting languages contained in configuration files that can be freely edited.