I believe it actually means 'The Survey Corps (調査兵団) is just going out there and (doing that thing called) playing at war, (aren't they?)'
It's all part of one larger expression,
Quote:ハンネスの言うとおりだ、まったく…壁の外にでる調査兵団って連中の気が知れねぇ…
勝手に戦争ごっこに興じてろってな
The って quickly implies ということ but wouldn't normally be translated into 'the thing called'.
The な has been covered, but I'll just add that it's just a masculine version of ね.
I believe that ろ instead of る is just the character's accent, so I read it as short for いる making the continuous action "興じている".
I might often think that ろ was short for ろう, but I don't believe that you can have a volitional ている expression.
I can see how you might think it's command form but I don't believe you can have a command form of a of a continuous-action expression either, although you can have a command form of a continuous-state expression (i.e., 黙っていろ) so... maybe. In any case, it seems weird to have a command not at the end of the sentence, I wouldn't expect a って or a sentence final particle after a command form.
Also, 進撃の巨人 is a very popular manga. If you give the volume & page number, you'll be giving all the context in the world, and you'll likely get a quicker and more accurate answer.
Edited: 2013-10-08, 11:11 am