Hi Daisy.
A wildlife pond certainly does not need a filter or pump.The latter is a death trap to amphibians.Certainly a half barrel can be planted up to look attractive with perhaps a miniature water lily and other small plants.It will then attract some insect life.However if you want to keep frogs,tadpoles and newts you are going to have to be very careful to get the slope into it just right.Strange as it may seem ,if newts cannot get out they will drown,and if frogs cannot they will starve.You will however be able to raise a few tadpoles in it if you can get hold of some spawn.Once the pond is mature and you have enough plants,weed and insects they will be fine,supplemented with a little flake fish food.
However once they start to lose their tails they will need to be able to climb out on to rocks to breathe air and then be able to get in and out as they require.They are very vulnerable at this stage.The surrounding areas of the pond also need to be well planted up,with logs,stones and damp areas for them to take shade and be encouraging to insects.
As long as you make sure that whatever you use to make a slope is secure and won't move,you should be ok.There is just one other thing to take account of.If you have any hedgehogs roaming about,they also need to be able to climb out if they fall in.A drowned hedgehog,bird or little mouse is a very upsetting sight.