Learn Turkish: Savaşçılar uzun kuşatma boyunca ölmüş olsa gerek. - The warriors must have died during the long siege.

This structure combines the reported past suffix -mIş with the phrase olsa gerek (literally "it is necessary that it be"). Together, they express a strong logical deduction about the past. It shows you're making a highly probable guess based on circumstances or evidence.

Could I use ölmüş olmalı here instead?

Yes, ölmüş olmalı means almost exactly the same thing. While -mIş olmalı is the standard, everyday way to say "must have [done]", -mIş olsa gerek is a slightly more literary or formal alternative. It carries a subtle nuance of "it stands to reason that they died."

Since savaşçılar (warriors) is plural, why isn't there a plural suffix on the verb phrase?

While you might occasionally see ölmüş olsalar gerek, the phrase olsa gerek is generally treated as a fixed modal expression meaning "it must be that...". Because it acts like a frozen phrase, it usually drops personal or plural suffixes, even with a plural subject.

What is the difference between using boyunca and sırasında for "during" here?

Sırasında simply means an event happened at some point during a timeframe. Boyunca (formed from boy "length" + possessive -u + adverbial -ca) emphasizes the entire duration. It implies the warriors died "throughout" or "all along" the long siege, rather than just in one isolated incident.