Colonial French town planners cleared many Ottoman buildings when they redesigned the Algiers waterfront and laid out what is now the place des Martyrs, but they left the Djemaa el-Djedid. Contrary to its name, the New Mosque, sometimes also called the Pêcherie Mosque, was built in 1660 on the site of an earlier Quranic school and paid for by public subscription.
The mosque is unusual for Algiers, built in a recognisably Turkish style, with a series of domes and vaults, although the minaret is Andalusian in style. It is also unusual for being designed as a cross: local legend has it that the architect was a Christian, supposedly executed for his trickery.
It has two entrances, one on the place and another on the steps of the ramparts, leading down to the port. Like most of the mosques in Algiers it's normally open to non-Muslims outside of prayer time, but it's worth taking a local guide with you.