They were crossed only by a drawbridge on the side facing the river, and the garrison could therefore obtain by boats supplies or reinforcements as needed from the town.
The Green and Blue Brigades at once commenced opening trenches against this fort, and would have assaulted the place without delay had not a number of boats been brought over by a Protestant well wisher of the Swedes from the other side of the river.

The assault was therefore delayed in order that the attack might be delivered simultaneously against the positions on both sides of the river.

The brigade of guards and the White Brigade crossed in the boats at Gernsheim, five miles from the town, and marched against it during the night.
The Spaniards from their lofty position in the castle of Oppenheim saw the campfires of the Scots around their fort on the other side of the river, and opened a heavy cannonade upon them.

The fire was destructive, and many of the Scots were killed, Hepburn and Munro having a narrow escape, a cannonball passing just over their heads as they were sitting together by a fire.
The defenders of the fort determined to take advantage of the fire poured upon their assailants, and two hundred musketeers made a gallant sortie upon them; but Hepburn led on his pikemen who were nearest at hand, and, without firing a shot, drove them back again into the fort.

At daybreak the roar of cannon on the opposite side of the river commenced, and showed that the king with the divisions which had crossed had arrived at their posts.