Dual and multi-classing gives you the advantages of both classes, and the disavantages of both classes. For instance: a fighter/cleric can only use cleric weapons. Even though you are technically a mage, you still have the restrictions of your previous class. If you notice, you cannot equip a sling either, as Kensai can't use ranged weapons (although they can use thrown weapons like daggers and darts). You can wear robes as a Mage because it is not considered armor.
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"This place will be your tomb."