Complex sentence: Although it was not built on particularly high ground, the inner Castle which occupied much of the area of today's castle, was not directly accessible to the attackers, because two other baileys had to be taken and secured before a final assault could be launched.
Split into simpler sentences: Although it was not built on particularly high ground the inner castle, which occupied much of the area of today's castle, was not directly accessible to the attackers. This was because two other baileys had to be taken and secured before a final assault on the inner castle could be launched.