Rachel. It is possible to get married on the wall. There are several permits that have to be gained in order to do so. There is also a fee (depends on the time of the year) that has to be payed to the city for the use of the space. There are also some requirements that have to be taken into consideration such as bathroom facilities,etc. It is very possible though. I recommend EVENTO & COCINA. This firm can help you with everything you need for your wedding. Venue, permits, catering, planning,etc. WWW.EVENTOYCOCINA.COM. They are great!!
As Chef said, you will need a couple of permits that I am sure that your wedding Planner will help you out, we have shot a couple of wedding in the "Baluarte" (Wall city venue) and i am sure that its preatty much on what you are looking to, check their wedding: