How can I maintain the calendar via iCal?

You get to the iCal function at the menu item rates & availabilities - iCal import/export. Here you can enter the iCal links. You also have the possibility to insert more iCal links, each one from other wesites, to keep your calendar up to date.

At the moment synchronisation is done every 3 hours. A manual update of the calender is in addition also possible.