Merging vertices

Hi Guys

Am learning Blender and I just made a tutorial about an medieval Arch. I have made one and duplicate it three time. As you can see in the screenshot, all the selected vertices needs to be merged to finalise the mesh. The only way I know is to merge them one by one, selecting two vertices and then merge them together then the same with the nest two. etc. I was wondering if there was a way to merge them using one command.

Select all the vertices (A) and W / Remove Doubles. Either at the bottom of the toolshelf (T) or by F6 you can then adjust the Merge Threshold if the vertices are not exactly in the same location.

Thank you sir, that worked