Is it hard?
One on top of two.
Do you mean hard (physical) work or difficult?
Not if you take your time. A good brickie with a labourer can do 1000 bricks a day. I peaked at 100 building my garage.
Take your time, use a string line, use a torpedo level to make sure each brick is level front to back. Watch your spacing and get the water level in your mix right, too dry and it's difficult to align the bricks, too wet and you'll get it everywhere. Use an additive, makes it a lot easier to squish the bricks down.
What are you planning on building?
Somebody all thumbs hasn't a hope. Somebody with some skill will probably be fairly slow , but should make some shape of it.
Read up on it also as there are various bonds which are suitable for the structure you are  building ie extension might need damp proof course, cavity wall and stretcher bond.
a wall around your moat might be English garden wall bond.
I have built a low garden wall (about 12 courses total, including the courses underground). Wall was in two parts approx 2m each, and each was a rectangle with plants in the middle. Fairly easy at that height - I suspect that the higher you go the harder it is, as any slight error will be magnified.
Start with a level base, and if one course goes slightly off level, bring it back on the next one. Front to back level is important, and seems harder to get right than along the wall. Keep checking that the wall is vertical. Remember to keep the joints in line vertically. You WILL drop mortar on the ground, don't worry about it. You'll also get mortar on the face of the bricks; brush off with a dry brush an hour after you've laid them, then wire brush next morning - anything left will then come off with brick cleaning acid.
I agree with stumpjon's advice, and I managed about the same rate - 12 bricks per hour average, including mixing the mortar etc.