Basically what i want to do is add a field of vision to some soldiers in a castle so that you have to avoid getting spotted otherwise you get thrown straight back to the beginning. Is there anyway to do this? and if so how? Any help at all will be much appreciated

been wanting to know this for quite some time now

Funny, I have the exact same problem. The best way that I know of by using the events, so far, is to set up the events so that the "spotted" events know where the "Soldier" is. 'Turning off' the event when the soldier walks onto it (thus being behind the soldier), and turning on the line of "spotted" events when the soldier looks at them. It's possible to make make it work by using a few variables/switches.

Another method is to have the "spotted" tiles follow the soldier, or essentially "Drag" the soldier.

1) All of the spotted tiles are "through", this allows you to move the events on top of each other when the soldier is looking at walls. (why it would do that, who knows)
2) The soldier event controls all of the tiles. (If the soldier moves left three times to go around a turn, all three "spotted" tiles do so)
3) When turning, you tell the soldier to turn, and all three tiles to move to the new spots in front of him (this will happen in an instant)
---However, using method number two (above) locked up my other events, so it's not a good way of doing this---

Both methods are a little clunky, but it gets the job done.

There was a script I saw once that did this... something along the lines of Guard Script or Roving Guard Script... lets see what i can find...

EDIT - I did not find the script, but i did find this tutorial on how to do it MGS style!! With a little rework, you could edit it to your liking.
http://uirpg.com/tutorials.php?act=view&tutid=42

http://rpgmaker.net/tutorials/128/

Here is a tutorial I wrote on having a field-of-view. If you have any questions then PM me. That's designed for RM2K(3) but it should work fine in RMXP once you work out how Enterbrain renamed the commands between versions.

There's this script which works pretty well. Although it's made the game crash once or twice for me, but despite that it's pretty good. I wish I could figure out how to fix it though.

Woah. Sorry for replying so late. After awhile nobody was posting any solutions so I just kinda forgot about this... Thanks everybody though. Although I no longer desire to add a field of vision for the time being. Sorry but I gave up after nobody was posting. However this could be useful in the near future so thanks again .