For such simple level design the blocks work fine. I would go to the effort of merging everything together and removing unnecessary polygons (sides of blocks which are aligned) however in order to keep the surface count down and to eliminate the cracks which appear between blocks. Just using a mesh would work too but would probably be slower to load.
If you want to do anything more complex with the level design you will almost certainly need to switch to modeling your level externally and loading it in as a mesh.
If you want to do anything more complex with the level design you will almost certainly need to switch to modeling your level externally and loading it in as a mesh.