The name 'Bastionder' is a portmanteau of the Dutch words 'Bastion' and 'Onder' (under). The project excavated part of the underground area between the bastion's walls and the old city wall, covering it with a weathering steel and concrete roof with some windows. One of the requirements for the Bastionder was that it should disturb the existing park as little as possible. To meet this requirement, the roof only rises above ground level enough to provide a visitors' entrance. The roof is also covered with grass planted with crocuses, making it look like part of the park.
