Polytunnel....Help please

Having just acquired my first polytunnel I am now wondering about layout.

It has had to sit inside the remains of an old building so it is end on to a north wall and side on to a west wall thus leaving the west side and south end fully exposed ( and the roof of course ). The floor is cobbles laid into the ground so drainage should be ok?

I'm assuming raised beds are the answer and looking for ideas on construction, depth etc.
Hopefully I shall grow a few tomatoes, early lettuce and bring on some seedlings.

Hope all you experienced 'tunnelers' can help.