A gradual advance can be traced in the architectural style of the round towers. The stages of their development were parallel to, and probably contemporaneous with, those of the churches and oratories[297]. Three periods of building have been distinguished. It is not until the beginning of the tenth century, says Miss Eleanor Hull, that we find the towers mentioned, and none can be dated earlier with certainty. They are later in date than the churches built by St Columba at Raphoe, Kells, and elsewhere, and they belong essentially to the period when the Northmen made their attacks on Ireland. Nevertheless, they continued to be built until the middle of the thirteenth century[298]. All these facts are of vast interest when we recollect those Lincolnshire churches which, folk-memory tells us, once served as refuges from the Danes. Normally, the round tower contained three stories, and, like many of the English towers before described, had the outside door placed high up in the wall, so that it was accessible only by means of a ladder. Similarly, the oratory of Kells has three small chambers, or attics, between its round barrel vault and the outer pointed roof of stone; these chambers could be reached only by the aid of a long ladder, and entered through a hole in the inside roof. Viewed in comparison with the other buildings of the period, the oratory represents a lofty structure. It has a highly pitched roof, and receives no light save what is afforded by two small windows. This building is assigned by Miss M. Stokes to the year A.D. 807[299]. In passing, we note that the round tower was probably developed from the beehive hut, and became specialized for a definite purpose, which we shall discuss in a moment or two. The beehive house, in its turn, was most likely evolved from the wattled dwellings of the ruder aborigines of Ireland. Professor A. C. Haddon cited authorities to show that the round towers structurally betray their pagan design, by their retention of string-courses which serve no useful object. The towers, he considers, are, in reality, derived from primitive wicker huts, circular in plan, and of a somewhat tall type[300]. We now inquire what purpose the towers were intended to serve. The answer, though given with a fair degree of decisiveness to-day, has been long in coming, and there still exists a minority of writers who dissent from the orthodox view, and who favour other theories, some of which are of the fantastic kind.