Building is 100% free form and this is video by Necromaniak, one of players in alpha tests:
...many hours later:
He built this by C.U.B.E. and 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.
The game is Camelot Unchained, "Dark Age of Camelot 2", PvP / crafting / building MMO. Or you can check it's IGN Wiki for quick intro and more pics + videos. Access now is not cheap, but they'll have open beta (free tests for everyone) in 2017.