You do not need the battle tower to use charms. To permanently add one:
With the Xbox version, just select a piece of equipment with one or more empty charm slots in the inventory and hit A; a menu will come up allowing you to permanently charm the item with any of the charms you have collected. In the equipment menu you can hit Y to add charms to equipped items (currently that is the only way to charm a bow).
On the PC right click a piece of equipment with an empty charm slot.
Casper should be walking around north west of Lovis' tower. From the enchanter, if you follow the mountain around east and north towards the entrance into the Maxos temple area, you should encounter Casper and a bunch of chickens running around. The white chicken is in one of Bellegar's shrines south of there (west of Lovis' tower).
Tagos' axe should be on the ground near one of the bodies.
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