One of the more interesting aspects of the revamped RvR system coming in 1.4 is that siege engines can now be placed anywhere in the RvR lake. In preparation I hopped on the PTS to figure out how they work. Read on for the details.
Once your keep reaches Rank 3, it gains offensive mobile siege weapons. They can be found on the extended platform in the inner keep (where applicable).
Just go up to one and right-click on it (you have to be close to it). If it is available, you will receive a 5 minute buff (upper left).
You will also get a new click-able button above your action bar. Clicking this will give you a targeting circle allowing you to place the siege cannon.
That's all there is to it, use them wisely. If destroyed, they do become available again at the keep.