Building is 100% free form, this is video by Necromaniak - one of players in alpha tests:
...and this is how it looks many, many hours later:
The game is Camelot Unchained aka "Dark Age of Camelot 2", PvP / crafting / building MMORPG by Mark Jacobs. Or you can check it's IGN Wiki for quick intro and more pics + videos. Access to game now is kinda expensive, but they will have open beta (free tests) in 2017, right before launch.
Player building tool in that video is C.U.B.E. and it will be improved, this is just alpha test example. Buildings can be destroyed - unless you build offline or in starting zones BUT there will be siege windows (time for destruction) and it should take big number of players and siege engines.