Steam Engine
I am unsure of the technology available to this society. However, steam engines are fairly basic machines. Further our current society has plenty of refined metal that could be used as the basis for redeveloping this technology. Assumably you could just burn wood in place of coal. This option would likely not provide as much power but it is viable. Finally, the steam engine could be mounted two or three floors up for defence -- you would have to rely on animal / human power to position the fuel.