In response to the OP question. There should be some measures of safety provided in towns, but no where in the game is going to come with a guarantee. When your offline an OPC will try to defend your body and property to the best of its ability, and may seek help from authorities or other sources.
In regard to the conversation about a "golden horde" scenario. Things to consider...
1) There are no guarantees that you will be able to buy army sustaining logistics with exposition points. Few of the proposed kits that we have seen so far include siege weapons, armaments, and logistical supplies. Other than the Military Kit, which is only available to Dukes/Duchesses. A la cart items will have limits. Furthermore exposition items will need to be retrieved from NPCs who have limited stocks. You may not be able to retrieve all of the things you buy right away.
2) Exposition is for getting setup. It wouldn't seem logical to allow conquest of any sort during the actual Exposition.
3) Assembling your body of hundreds of players will take time because its unlikely that you will be able to get a large number of players spawned into the same place. Which means they have to survive travelling in order to assemble. On top of this its unlikely your going to be able to claim enough combat skilled personnel within close proximity.
4) If your able to destroy a settlement, without a Casus Belli, you wont be able to take ownership of the land. This means NPCs and OPCs wont recognize you as a legitimate entity and will probably not engage you with trade or industry. That will put significant limitations on your ability to sustain yourselves.