Round table

I'm not entirely sure, but based on experience, if you have, for example, 7 stacks of fairies, and there are fairies remaining in the garrison, they will combine into a single stack when attacked. If the garrison contains individual units, and you're attacking with, say, an angel, they will also combine into one stack. However, if you leave creatures of the same tier and divide them into several stacks, without leaving space in the army for garrison units, only those 7 stacks will fight. In general, if you're interested, you can test this yourself; it shouldn't take too long.