I think a dike would be a lot better than a wall, since it gives us altitude and thus knowledge of wider swathes of territory (if it's built high enough, of course), and could easily be made defensible with the addition of fortifications, along with the benefits that dikes grant (higher volume of water and faster running water in general---harder to find fords and thus more difficult for would-be besiegers). But it's up to you all. 

[@Shorticus] And thanks for posting that little tidbit about Eliz, I wrote that post while I was in class (lel) so I didn't have a lot of chance to look around. I've edited some of the errors and made it a bit more readable as well. 

What does everyone think of Dunlad's proposal?