Late 12th Century Northern Cog
In the conquest of Livonia, Bishop Albert took an important decision when he moved the centre of all his operations to Riga, close to the mouth of the Dvina, which was strongly fortified. Castles were the keys to the land, and time and time again resisted pagan assault. In 1211, a large Estonian army supported by a fleet attacked Caupo’s great fort on the Aa, which was partly held by Christian Livonians; they were able to hold out until a relief force from Riga arrived.29 But not only were the forts of the Christians stronger – they had also developed the art of siege warfare much more highly.
The decisive technical factor which sustained the German conquest was not military at all, but a by-product of the economic expansion of Europe, the merchant cog or round-ship. These were large, solidly built craft up to 30m long and 9.5m wide – by the end of the thirteenth century, deadweights of 200 tons were possible. They were popular with merchants in the Baltic and elsewhere because they could carry an immense load, up to about 250 tons, with a crew of only about 18–20, and so they were available to the German crusaders. By contrast, their pagan enemies did not have the means to build such expensive vessels. The cog could carry 500 people – very useful for bringing in and supplying crusaders and their settlements. As fighting ships, they were helped by a high freeboard, which meant that they towered over anything else that could float, like a castle at sea: indeed, one was turned into a floating fort to guard the mouth of the Dvina against enemy attack. The cog was never fast or manoeuvrable, but it was very seaworthy, especially after the introduction of the stern-post rudder, and it could be towed up rivers. Henry the Lion came to recognize the need for seapower, as we have noted, but the increase in Baltic shipping brought a decisive advantage to the Germans. In 1215, two cogs were enough to scatter the Estonian fleet in the mouth of the Dvina, and a little later that year nine cogs bearing pilgrims returning to Germany were trapped by bad weather in Oesel, where large numbers of Estonian ships tried in vain to destroy them.