When you summon a creature, what determines its place in the three creature slots? I've noticed that the game tends to place creatures with defender and reach towards the first slot, but I can't seem to completely figure it out, occasionally causing a creature to be killed prematurely. Sometimes even costing me a win, or challenge objectives.
In my experience, any creature that you summon will be placed at the top of the pile. Creatures with Defender are placed at the bottom of the pile, unless one is already there, in which case the new creature is placed above the last creature with defender.
If you are replacing a creature the new creature takes the spot of the replaced creature. I haven't tested what happens if you replace a non-defender creature with a defender one.
Flying doesn't seem to affect the order. I'll do some more research to see what affect a creature having vigilance makes.