Manifolds, Structures Categorically
  A notion of general manifolds is introduced. It covers all usual manifolds in mathematics. Essentially, it is a way how to get a bigger 'fibration' over a site which locally coincides with a given one. An enrichment with generalized elements is regarded which allows to see hom-sets of a given category as (almost) objects and to transfer some technics from objects onto hom-sets. Lifting problem for a group action and actions of group objects are also included. 
G.V. Kondratiev