The following buildings in Stronghold 2 can only be placed using the map editor. Note that you can also create your own ruins using the editor as well.
A Ruined Monastery can be placed using the in-game editor. It cannot be placed while you are playing your map. Once playing the game, Monks will arrive and attempt to rebuild the Monastery. You will need a supply of Stone in order for them to complete the Monastery, which the Monks will take from your Stockpile. Placing a Ruined Monastery is a good idea for a mission objective in a custom map.
A Ruined Bridge can only be placed using the in-game map editor. Once playing the game, Monks will arrive who will attempt to rebuild it using Wood and Stone from your Stockpile. Once completed, the bridge will be fully passable by units and will behave like any other bridge placed on the map through the editor.
Warning Beacons can only be placed through the in-game editor. Once playing the game, Monks will arrive to attempt to rebuild it using Wood from your Stockpile. This picture below is what the beacon looks like before it has been build.
Destroyed Ships can only be placed through the editor, and you can change the orientation of the shipwreck by scrolling with the mouse wheel. It is also possible to script events during the game to create your own shipwrecks. You can view more information on how to configure ship events in Stronghold 2 here.
Birdhouses can only be placed using the in-game map editor. They can help with the scenery of your maps, especially when placed near farms or in villages throughout your map. They generate +1 Honour every month to the estate which they are assigned to.
Outlaw Camps can only be placed using the in-game map editor. Once built, they will periodically generate Outlaws in the game, who will attack any player who goes near them. They will also sent droves of Outlaws over to your Castle to attack you as well, and will harass the nearby village estates of players too. The only way to stop this is to send your troops to destroy the Outlaw camp.