Each client player has an attribute block and the server also holds such an attribute block for every player. All attribute blocks the server holds are included in the save game. The client and server synchronize their blocks. The server sends it's block to the client at game start or reload. The client sends an updated block at each end of turn to the server. Since the maximum packet size is limited to currently 4k and the attribute block can have arbitrary size (although limited to 64k in this initial version) the attribute block can't be transferred in one packet. So the attribute block is divided into attribute chunks which are reassembled at the receiver. No part of the server knows any inner structure of the attribute block. For the server an attribute block is just a block of bytes.
Since an attribute block isn't a good user interface the user can access the attributes through a mapping/dictionary/hashmap/hashtable interface. This hashtable will get serialized to the attribute block and the other direction around. The key of the hashtable consists of: the (real) key, x, y and an id. The (real) key is an integer which defines the use and format of this attribute. The values of the hashtable can have arbitrary length. The internal structure of an value is unknown to the attribute handling.
For easier access there are wrapper functions for the common types unit, city, player and tile. So there are easy methods for attaching arbitrary data to a unit, a city, a player (self or other) or a tile.