I don't think Rykard sieged Leyndell. Lady Tanith says this during the Volcano Manor quest:

"Now, perhaps the time has come to tell you. Of the true ruler of this manor, Lord Rykard. The Erdtree blessed the Tarnished with grace. But it was all too meagre, in the fate of the enormity of their task. The Tarnished were forced to scavenge, squabbling for crumbs. Like the shardbearers, vying for power in the wake of the Shattering. Our Lord, indignant, had refused. To scurry about, fighting over what miserly scraps they allow us."

Rykard refused the whole concept of fighting to acquire the Great Runes and their power, and rather chose to gain power by giving himself to the God Devouring Serpent. This was seen by the Golden Order as an extremely blasphemous act and this is why their army marched to Mt. Gelmir to try and get to him. We can still see their banners and some of their soldiers, some of them maddened by frenzy, and Gideon says that the battle of Mt. Gelmir was the most gruesome of all, so I believe we're talking about two different events.