Defining a map's borders - how?

Hey guys, Sorry ahead for the n00biness, but let's say I have a map of mountains and planes, and I made that map out of a single drawing I had. I only want my players to be able to walk on the planes and not on the mountains (naturally). How do I do that? And I did look over the documentation before posting here (even googled). Couldn't find the answer. I will appreciate a reply.

Without using the API? You can't. With using the API? I don't know if it's possible, but it might be. I don't use it so... :::Shrug::: -Phnord.

I would be inclined to tell my players "These are mountains. You can't walk over them." Just kind of the same way you don't expect players to try and walk through walls (well, most of the time. Damned phasing.)

It is possible to use dynamic lighting lines and the API to prevent players from moving their tokens through, but it's buggy and doesn't always work. especially if they just mash the arrow key down.

If they insist on going through the mountains, make random monster rolls and enforce the movement rules for your system or just tell them not to but that will backfire on you because most gamers will do what they are told not to do. In reality, the API , as others mentioned is your best bet for limiting your players choices. Your desire to keep the players on the plains and not in the mountains should be dealt more in game then outside the game unless you want to sit down and tell the players you're not ready for them to travel in the mountains. Most gamers will respect that and stick to the plains then you can work out why the characters would stay in the plains. Now this bit above is just my opinion and you can try it or not but its your and your group's game so it is all in your hands on how you wish to deal with it. Have a good game.

It's ok guys, I just thought I'm missing something in the toolset. If it's something you have to instruct the players, then I will. I appreciate the replies. Cheers!