Built between 1668 and 1671, this domed church has the curious distinction of being the only round Protestant church in the country. Falling attendance forced its closure in 1936. Although it's not open to the public (other than for conferences, trade fairs and events), it's connected by a tunnel to the neighbouring Amsterdam Renaissance Hotel, which you can contact about visiting if the church is not in use.
Ironically, the old church on the Spui, which the Ronde Lutherse Kerk was designed to replace, is still in use.