The Abbaye de Saint Étienne (Saint Stephen's Abbey) in Caen is also known as the Abbaye aux Hommes (Men's Abbey), to distinguish it from the Women's Abbey close by. If it looks a bit like an English cathedral, you're on the right track – this stunning example of Norman Romanesque architecture indeed served as the inspiration for so many churches on the other side of the Channel. (Although keen-eyed visitors will notice the Gothic apse, a sign of the church's architectural evolution.)
There are two highlights of the Men's Abbey; the first is the tomb of William the Conqueror, whose mark on Normandy has never been forgotten. The second is a bit of a hidden gem – the cloistered gardens, accessible by going through the town hall. It's another world inside there, and a favorite with photographers.
The Abbaye aux Hommes has varying open hours, so it is best to consult its website before visiting. Hours are dependent on the season and the regional school calendar. The former monastery is sometimes open for individual exploration and guided tours, although it is, of course, closed when religious ceremonies are in progress.