Why should I keep all 5 army slots filled?

You can deploy up to 5 armies at once. A common beginner mistake is using only one or two and leaving the rest empty.

Two reasons to fill them all:

1. The more waves you send, the more big targets break. Damage accumulates on a target (which never heals), so chaining armies onto a stubborn tile eventually cracks it.
2. You don't reconfigure an army in the field. Swapping dragons or troops happens at the castle. So you prepare set armies in advance and send the one that fits the situation.

The good habit: keep a fixed set of 5 ready armies (say 2-3 "long fight" armies and 1-2 "fast burst" ones), and keep them all full of troops.