The power bar replenishes over time and contains an ascending series of buttons to press. These, starting from the lowest are: * Arrow volley * Heal troops * Summon small group of spearmen * Summon small group of engineers * Get 2000 gold * Summon small group of knights * Rock volley Using a power diminishes the bar, the extent on this depending on the level of the power in the list. Stronghold Crusader Extreme is usually much more difficult if you do not use the powers, so not using them at all is a good idea if you want a challenge.