I'm using simultaneous initiative (with an action declaration phase) and one-minute combat rounds for In the Realm of the Nibelungs, both a first for me and my group. Combat mostly runs via Swords & Wizardry Continual Light, a D&D 'retroclone', albeit with some changes (such as simultaneous initiative).
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In the Realm of the Nibelungs has begun! My consultation with Ron can be found at Adept Play here.
The game is set in 5th century Burgundy (as portrayed in the medieval German epic The Song of the Nibelungs) and uses a 'homunculus of (old) D&D' I crafted for this purpose.