While the game is pretty casual and intuitive, some things aren't explained...
The arrows on the screenshot above show two options. The trash can allows you to dismiss the current troop configuration and select the units yourself. Choosing your own units increases the difficulty level of the game. The little guy with a plus next to him allows you to choose a default troop configuration.
Another thing that isn't obvious are treasure chests floating between islands. Grab 'em for extra loot.
Strategy tip: each island has a treasure chest. Sometimes it's better to just capture the treasure. From that point you can click Pause and choose to return to the map and/or play the level over. You don't lose your loot if you take a do-over, but enemy troop strength may be greater.