As u/TheWellspokenMan said the Germans were very prepared for the inevitable invasion. I will not reiterate what he said because he is spot on. The Germans knew first hand that dangers of Airborne forces. The Germans had successfully used Airborne assaults earlier in the war, such as the 7th Flieger unit landing gliders on the roof of the Belgian fortification Fort Eben-Emael . The German Fallschirmjäger (Paratroopers) took the much larger, well dug in Belgian garrison by complete and utter surprise. The Germans did suffer some casualties, but ended up taking the entire fortification and capturing its garrison in a very short amount of time. The German Airborne assault on Crete was a mixed bag. It showed that it was possible to land a larger Airborne force… but at a very heavy price. That was the last major Airborne assault by Germany. The Germans did use their Fallschirmjäger troops throughout the war, but more as a hit-and-run or rescue style Commando force; so small scale operations. And at the end of the war the name Fallschirmjäger meant nothing more than you were an infantry unit in the German Luftwaffe, but most likely never jumped out of a plane, and were not an elite unit but a clerk or mechanic given a rifle.

Back to your question OP, the Germans knew the dangers of Allied paratroopers and commandos landing somewhere in Northern France. So for miles inland from the Channel Coast, the German’s Atlantic Wall was littered with a mass variety of defenses for seaborne and airborne troops. However, the Germans did not know when or where the Allies would attack. The Germans thoroughly believed that the Allied invasion of France would be in the North Eastern part of France, specifically the Pas de Calais department. For months the RAF and USAAF were bombing and strafing military fortifications and industrial areas of Calais. The French Resistance activity was ramped up in that area – blowing up bridges, cutting telephone lines, blowing up rail lines etc… German double agents and Allied intelligence services were feeding mass amounts of misinformation to the Germans intelligence organizations, in what was dubbed “Spielfunk” (Radio Play) – the cat and mouse game of espionage and subterfuge. Also General George Patton, whom the Germans had a high respect for, was given command of the “Ghost Army”, the fake and inflated army garrisoned in SE England across from Calais. The Germans were 90% convinced that when the Allies launched their invasion of France that it would happen in Calais, not in Normandy. Hence why Normandy was so lightly garrisoned and didn’t end up in utter disaster for the Allies like their previous attempts at Salerno and Anzio in Italy (where Airborne units were dropped behind the lines before the seaborne units landed).

The Germans were ready. But they didn’t know when and where the allies would attack. They knew the allies had a habit of dropping paratroopers and gliderborne infantry units behind the lines the night before the main D-Day landings by seaborne infantry and armored units. The Allies had a proven track record of this. So that strategy was not a surprise to Germany. The allies had done this 3 times before on a larger scale in Sicily and Italy. The Germans were not dumb. They knew the allies would open up a new front in France, especially since the “Soft Underbelly of Europe” Italy was not proving to be an easy route in to Germany. The question was “Where and when in France?”. And through great trickery the Allies had the Nazis convinced that the Allied invasions was going to be in the Calais region, and not in Normandy. Even as American and British Paratroopers were scattered to the wind across Normandy, and attacking the Germans wherever they saw them, the German commanders believed all of this was a diversionary attack meant to shift focus away from the real invasion; the invasion in Calais. Whoops!

Yes, the German defenders in the Normandy region were well aware of the possibility that paratroopers and glider borne infantry would be used in the event of an invasion. Special defences were set up specifically to combat these. I don’t know if the AA batteries in the Normandy region were positioned specifically for use against Allied transport aircraft or if they were stationed there for a general anti-aircraft considering the heavy bombing campaign the Allies were waging throughout the Normandy region. However, the German defenders flooded the fields inland from the Normandy beaches both to slow down any Allied advance in the event of a seaborne invasion as well as to prevent those fields from being used as potential landing sites by paratroopers and gliders. During the event itself, the disorganisation of the transport planes saw many green lights given over these flooded fields resulting in many paratroopers drowning under the weight of their gear or tangled in their parachute webbing.

Additionally, in fields that were not flooded, the Germans erected wooden posts that were several metres tall and buried so that they stood vertically upright. These dotted Normandy fields and were nicknamed *Rommel’s Asparagus. The idea behind them was that they would prevent the fields from being used as landing sites by Allied gliders as the sturdiness of the poles would easily tear through a glider attempting to land. Some poles were also mined just to add to the danger they posed.

Such defences only proved marginally effective as they were placed too far inland, the German commanders believing that any airborne operations would come further inland then they actually did. This allowed the Allies to concentrate their glider borne forces and paratroopers in areas that while potentially flooded, did not possess the heavy concentration of wooden pole defences that were found further inland.

Of greater danger were the pre-exisiting bocage and hedgerows which bordered French fields. Typically characterised by an earthen embankment topped with hedges, these proved particularly dangerous as the dense root systems formed the hedges into an almost impregnable barrier especially against lightweight gliders that had no means of arresting their speed once they had landed. So dense were these hedgerows that Allied tanks were unable to force their way through until an ingenious solution was found by welding segments of metal beach obstacles onto the front of the tank, allowing them to dig the roots out as they passed through. The density of the bocage and hedgerows were the bane of the Allied Expeditionary Force after D-Day and were the chief reason why the Allies were so slow to seize and clear the Normandy region after the landings.