Change Query Arguments (filter) with jQuery/Ajax or PHP?

The permalink structure you’ve described above e.g…./hotels/hamburg/features/sea-face isn’t really something that could be achieved by using custom fields. You would be better off creating a new taxonomy called ‘features’ and populating it with terms like ‘sea-facing’ and any others.

In order to do queries for multiple taxonomies I would recommend the excellent Query Multiple Taxonomies plugin by Scribu. This plugin will let your users build searches that span across more than one taxonomy e.g. city AND feature.

You could either use the filter widget that comes with the plugin or build your own if that doesn’t quite cut it. Think in the long run this will prove simpler than messing around with more complex ajax requests or complex wordpress queries looking for combinations of custom fields.