use type s mortar and pin the caps to the block wall,
type s for the leveling bed and head joints between the caps
drill 2 holes on the bottom of each stone near the end and use either 3 inch tapcons or simmilar type concrete anchor. dab a bit of adhesive on the end of each screw and place the cap temporarly in place, the adhesive on the tips will mark your holes and then drill them slightly bigger fill the holes with adhesive or epoxy them place mortar then set cap. caps should always be pinned outside or inside where anybody can sit on them or lean up against them. if you look at most planters that arnt done like this the joints are cracked for sure.
hope this helps