Keep this as a rule of thumb about sniper decks: If there's going to be a sniper deck, there's going to be more then one sniper. I call it a strange day whenever I join a 2fort server without limits and there isn't 6 or 7 snipers on each team... even though the deck is really only meant for two.
For an arena map, I'm going to strongly suggest you don't make a sniper deck unless it's very very open or there's more then one way to get up there (and as it sounds the latter isn't going to be possible). However much I dislike the map, take a look at arena_watchtower. There's two circular staircases that spiral up to the top. You can easily replicate this except on a smaller scale. Also, if your map design allows for it, you can make an "Operation Overlord-ish" type bunker that overlooks the center area, like what you see in almost any WWII movie that takes place during D-Day.
However, as was said above, I would strongly suggest against any sort of ladder or elevator, as both break the flow of movement in TF2 and they do just look silly when used. Plus, moveable objects that players can stand on don't work well in multiplayer source engine games.
EDIT: FAST MOVING FORUMS UGGG. It's what I get for starting this and not finishing it till later. Looks like most of what I said was said before.
