People are surprised that mud buildings can last for many years, even in England's wet climate. There are still some mud huts standing that were built in the 1600's. It is not so difficult to build excellent garden buildings, walls etc using this historic technique. We built this one for a television program. It is made using soil from the field next door.
The important thing is to give the walls a dry base. We made a mini stone wall, so water doesn't seep upwards and to give the roof an overhang to keep the rain away. Because they are made of local materials they blend naturally into their surroundings. We made it in 600mm layers of mud mixed with straw and lime - rather like a huge mud pie.
Historically, they used cattle to tread and mix this. Each layer was left for a few months to dry before the next was added. Mud structures take little skill to build and some primary schools have got the kids to build playground dens from mud. You can paint the mud walls with subtly-coloured lime washes if you want them to stand out more.