That is not a proper moat... its a stream you have dug that exits into the sea. Therefore the troll can get through still, along with anything else really. Troll head is above land, so it is not deep or wide enough yet. Thanks for posting picture though.
To defend against that situation, need to dig wider, and deeper. Even then its a pain building near water edge. Sure it looks good, but not enough depth for a good moat. You are also going to need a wall, either a wood, or a raised one.