While obviously you'll need the Codexs if you actually want to play LATD, I find it useful to look up an army on army builder first to get an idea of their 'strengths' and how they function. At least that's what I did when I was first learning about 40k, it gave me an idea of whether to buy certain codices.
So basically I'm saying, you may find it useful to play around with LATD on Army Builder, so you get an idea of how they work.
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