You negative nelly, JoshF.
Worth remembering: by medieval times, people had already been brewing ale for millenia. They weren't a time that lacked for knowledge or craft when it came to brewing, and what they brewed then must have been every bit as complex and sophisticated as what is brewed now - more in some ways, because they had knowledge of spices that somewhat fell out of fashion at some time between now and then, and some of that knowledge must have been lost.
At any rate: pretty sure molasses is a post-medieval thing, since it derives from sugar cane - ie, associated with that time of the slave trade when the UK became involved in sugar plantations around the world. Fair to say it was a cheap ingredient - it pops up a lot in household recipes and army and navy recipes. The recipes often include spices: all spice, cinnamon, cardamom, nutmeg, ginger. These would help to give body and flavour to the final product.
So, some tips. Andy, look around the net for old 18th century recipes with molasses: get an idea of the herbs and spices they used and the variety of things they did with it. Get the finest quality molasses you can find, with the best flavour; not everything will get fermented out. Try adding other fermentable sugars too; honey, especially a good organic variety, would be excellent (bear in mind honey may lengthen fermentation times somewhat because of the complexity of the sugars involved, but that's kind of what you want - it'll produce a more complex ale). Don't expect to get something brilliant first go: but aim to make it better each time and you should come up with some very interesting results. I salute you, sir!