I'm currently doing background research for a writing project, but I've not been able to turn up much online about motte and bailey castles. As such, I have a few questions that I would be grateful if someone could answer.
are there any good books available on this subject?
how thick could a palisade wall be made? Were they ever made from alternating horizontal and vertical beams?
is there any evidence of European attempts to fireproof the walls, either through plaster or partial charring (as I believe was done with some Japanese castles)?
is there any evidence for having two palisades separated from a short distance (like on some Maori pā), either by themselves or filled with earth?
is there any evidence of wooden walls with arrow slits?
Here are a selection of articles from the professional society for medieval military history (De Re Militari):
Jansen, W., 'The International Background of Castle Building in Central Europe', Danish Medieval History: New Currents, eds N. Skyum-Nielsen and N. Lund, (1981).
O'Keeffe, T., 'Medieval Frontiers and Fortification: The Pale and its Evolution', Dublin City and County: From Prehistory to Present, (1992).
Speight, S., 'Castle Warfare in the Gesta Stephani', Chateau Gaillard XIX: Actes du Colloque International de Graz, 1998, (2000).
Whittow, M., 'Rural Fortifications in Western Europe and Byzantium, Tenth to Twelfth Century', Byzantinische Forschungen, v.21 (1995).
Although in reference to later medieval castles Abigail Wheatley's monograph is a fascinating exploration of castles from the non-military perspective:
The Idea of the Castle in Medieval England, (York, 2004).
Finally to round off with a couple more general articles which may be of interest from De Re Militari:
McGlynn, S., 'The Myths of Medieval Warfare', History Today, v.44 (1994).
Mondschein, K., 'Strategies of War in Westeros', (2013). [I disagree with the author in general, but I think he makes some pertinent points about building an internally sensible world]
The book you want is 'Timber Castles' by Robert Higham and Phillip Barker, published by Batsford, London 1992, a massive study of motte and bailey and ringwork castles in UK.