The Lightning Tower Update (increasing the splash effect to 8 units) has drastically affected the balance of the campaign. I submit that it is now impossible to get beyond Z Day with anything less than a Castle Level 6 army.
Z-Day was once possible with Castle Level 3 troops (70-80 unit mix of lvl 2 samurai, lvl 1 archers). Now, I am struggling with a (nearly) maxed Castle Level 5 army (135 unit mix of lvl 2 samurai, lvl 2 archer). I'm still waiting on the lvl 3 samurai upgrade, but sincerely doubt that the 17hp bump from lvl 3 will prevent the towers from obliterating my samurai.
So, Z Day seems impossible to complete with anything under a Castle Level 6 army. I'll check in when I'm fully upgraded, but until then I hope someone can prove me wrong.