The shaman's mana bar is the long bar behind the shaman's icon. It's divided in many small vertical bars. It will interact according to your mana. Mana is the only resource in Populous aside with wood. It's really important to charge spells or to train units. In every stage of the game, the mana is split 50% for units training and 50% for spells charging. This means if you have no spells charging, your units will train faster; on the other hand, if you have no units charging, all your mana will focus on the spells, so you will charge them twice as fast; If you have no spells charging and no units training, you will be wasting mana, and the mana bar will flash red as a warning . this shoud never happen.

 

To understand this better, i give you another example.: if you have 100 braves and are only charging blast, your mana bar level will be on a high level because its a cheap spell that takes few mana to charge, so your mana levels will still be high. If you have 100 braves but are charging the spell volcano, your mana bar will be way lower than if you were charging blast, since that spell needs more mana to be charged - in this case, if you now started to traing braves into warriors, the mana bar would decrease even more, because you are doing many things at the same time that are expensive (charging volcano and taining troops).