Complex sentence: They were named "Habelli" for the ancient Germanic name of the river Habula (Modern German Havel) The name for themselves was the "Stodoranie", who built their main fortification at "Brenna" (Brandenburg an der Havel).
Split into simpler sentences: They were named "Habelli" for the ancient Germanic name of the river Habula (Modern German Havel) The name for themselves was the "Stodoranie". They built their main fortification at "Brenna" (Brandenburg an der Havel).