d'ailleurs

Meaning

Adds one more piece of evidence FOR what was just said: it is reinforcement.

Position and punctuation

at the start of the sentence · a comma after

Example

Cette solution est la plus simple ; d'ailleurs, c'est celle que la plupart des villes ont retenue.

This solution is the simplest; besides, it is the one most cities have chosen.

In the DELF

the B2 essay · the B2 oral presentation

The trap

D'ailleurs does not change topic: it drives the point home. If the point is new, write par ailleurs.