Although the Germanic tribes… they retained many of their customs and ways of life.

The correct answer is ultimately. The sentence implies that despite some initial changes or influences, the Germanic tribes, in the end, retained their customs. 'Ultimately' means 'in the end' or 'after a process of time or change,' which perfectly fits this context of eventual retention. 'Extremely' describes a high degree or intensity, which doesn't convey the temporal or conclusive aspect needed. 'Indispensably' means essentially or necessarily, which also doesn't fit the contrasting idea of retaining customs despite other factors.