I don't really test maps much, but when I do, I try to break them or look towards esthetics. I didn't test it for balance because I don't play halo competitively. With that in mind I can give you some of MY ideas and opinions.
I like the idea of a arena type map based on a normal forest side and an icy patched side (if thats what you're going for based on the structured walls merged with the rocks). I can get out of the map several ways, as well as seeing outside of the map. The weather seems okay though. You might want to consider less grass and more of just little patches of grass due to some lag.. grass makes maps terribly laggy (and trees). You might just want to polish on it and consider more time on the map to make the rocks seem more natural rather than just a big boulder sticking out of the ground. I also noticed being under the middle of the big hill I got stuck. I do like the look though, you could add some wind if you want for the snow perhaps. You may consider adding in some riot or explosion sounds to make me feel like I am in an arena for more emersion. Just throwing ideas out. Overall I see the basic map idea, and I like it, but it needs some little more polish and some love, as it feels incomplete.

P.S. delete some of the grass around and underneath the map. I hate when I run out of blocks when I am making a map and realize some objects are out in space. Happy forging!