What if they built a really big wall?
No...on second thought its been tried. Didn't work
[SouthPark] "Goddamn Mongolians! Every time Chinese try to build city wall, goddamn Mongolians try to break it down!!"[/SouthPark]
Seriously though, as Matthew Craw said, wouldn't a sizeable network of fortresses serve to delay the Mongols and deny them the advantage of mobility.
Warning: off the cuff, unsubstantiated argument ahead
This would be a defense in depth strategy where the fortresses rely on each other for support. This way the Mongols would be bounced around the fortress network, never really able to make any sizeable gains.
Of course this would take a level of cooperation and coordination that I doubt Germany was up to achieving but maybe it would work?