Sieges in Camelot Unchained will take time, resources and guilds/Realm coordination; keep trading will be strongly discouraged, and losing keeps/stabilizers will hurt. Territory control and Siege warfare revolve around building, repairing and destroying structures (based on physics), together with crafting and transport of siege engines.
Collision detection will be very important factor in siege warfare, due to potential bottlenecks and use of shieldwalls, turtling and other formations which can slow zergs and reward good teamplay. Also, loot & pillage of enemy buildings will be possible.,
Mark Jacobs: "Expect a wide range of siege equipment, and maybe some funny ones as well."; The siege ammunition mentioned so far are "Giant flaming rocks and maybe even giant flaming Dwarves!?" 
Videos and Sources
- The video below is The Opposition Podcast with Mark Jacobs; here (the first post in that thread) is the list of topics, in order in which they appear in the video - skip to 32:55 for Sieges:
- Mark-A-Thon Part 9 (video is inside; skip to 1:57 for siege engines)