I have no idea why this is so; my guess is that the "missing" tower was damaged at some point and the top layer of stones removed. If YOU know the answer, please contact me and I'll update this page!

I wasn't sure whether the towers were missing crenellations or castellations, so had to research. Turns out, a crenellation is specifically the "up/down" bit (typically used by archers to hide behind), whereas castellation refers to any form of fortification (i.e. making it castle-like). So in practice almost all crenellations are also castellations, but not all castellations are crenellations. More specifically, each "up" bit of the crenellation is a merlon, with the gaps between being the crenels (or, when designed for cannons rather than archers to fire through, embrasures.