Your way to 603qm in Darmstadt
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Coming from the Highway (Autobahn) 'A5' you leave the Highway at Darmstadt Centre ('Darmstadt Stadtmitte').

Your're on the correct way, if you are crossing a bridge that is going over some railways for trains.
Keep on driving the main road, that is called 'Rheinstrasse' (for about 2-3 Minutes and -- I think -- 4 Trafficlights) until you're entering a city-tunnel. Follow that tunnel and keep on the left lane. Then you get a left turn (still in the tunnel, at the end). Follow that road and try to continue the circle (counterwise).

At the first trafficlight after the tunnel you're crossing some tram-railways. You turn left at this trafficlight (again). Now the road is going a bit "downhill". Keep the middle lane, when it turns from two to three lanes.

Coming to the next crossing, you might recognize a manor-house ('Schloß').
After the crossing/trafficlight go directly to the very right lane and take the curve to the right -- keep slow because you close to our place.  (The street is going up a bit, a big building slot is on your right)

You see an old machinery-building with a yellow box in the front. Two arrows are near the top of the building. One of it shows you the way to the (second) entry on the backside of the building.