Well, at the end of one of the trailers, we see Alucard with a sword. So, the siege has to be after MoF. It's likely that after the fight with Simon and Alucard, Gabriel recovered from the fight and the castle resurrected itself. For all we know, the siege took place decades, if not centuries, after MoF.
Keep in mind that according to the narration of the first stage in LoS1, it takes place in 1043 AD.