Considering this is quite a dark, narrow strip along a walled area I think a combination of Asarum europaeum (evergreen and low-growing), Polystichum setiferum (evergreen), Geranium, Hosta, Helleborus niger would look lovely. Epimediums would be a possibility and also Campanula ( the small varieties). I would also consider putting in a climber; there are some lovely Clematis' that do well in shady conditions. Another thing I always like very much is using bulbs like Cyclamen coum, Corydalis, Fritillaria meleagris, Galanthus and Scilla. This way you'll have something to look at all year round.