Complex sentence: The first assault, mostly by Royalist troops from local garrisons, initially drove Blake and his troops into the castle, then tried to starve out the garrison, until the town was relieved by a force under James Holborne on 14 December.
Split into simpler sentences: The first assault, by Royalist troops from local garrisons, initially drove Blake and his troops into the castle, before settling into a siege intended to starve the town into submission. The town was relieved by a force under James Holborne on 14 December.