A tower or chamber that houses bells.
A tower moves into your head
Old French
berfroi
‘movable siege tower’
into Middle English
Middle English
berfrey
‘wooden siege tower on wheels’
became English
English
belfry
‘a bell tower’
The bell was a later association; the head is a metaphor.
Middle English berfrey could name a siege tower. Tower and shelter uses broadened, and association with bell helped shape belfry. A later figurative use names someone’s head, as in bats in the belfry.
A changed spelling can make an older word look like a familiar compound.