Surprisingly difficult and addictive. You'll probably get frustrated if you jump right into it, because this one is the sequel, and assumes you already know how to play. Play World Builder 1 just for a level or two so you get an idea what's going on. You'll need Macromedia Shockwave; plain old Flash won't do it.
The music is obnoxious as heck, turn it off from the "Menu" (upper right). The window you view the map through is way small and there's no way to zoom. You can pan around with the arrow keys instead of clicking the black arrows around the edges of the window. It's still annoying, but better than trying to use the mouse. There's a (postage stamp sized) map of the whole level in the upper-right corner just beneath the menu button. You can click on it to move the viewpoint rapidly.
The level pictured above is level 10, "Crossing the River". I spent about two and a half hours on it without even noticing! There are several really sneaky little tricks you'll need to use to win this one. Unfortunately, they don't seem to give you anything special if you get all the bonus goals. This would make a pretty good PSP game, or even good console game if it were revamped a bit for better visibility. Surprisingly deep for a flash game.