A base was made for the bridge which fits inside the walls. This will act as a template for the plan view and as a slightly raised internal floor. Seen here at left. It was originally T shaped but evolved a bit with the stacks moving outboard a bit and a 'bow window' being added to the rear (top here).

This then formed the template for a ceiling which is 10mm larger most of the way round to allow for the outward slanting windows. Seen here at right. This has cutouts for high level / high angle windows and to allow access into the bridge for detailing.