I recently attended HMGS's Historicon and participated in the Viking's siege of Paris. It turned out that they were using a rule set I had already experienced in play at a previous Con. The first time was in a presentation on introducing wargames in the classroom. At the time I had thought the rules were homebrewed because everything was condensed onto a single reference sheet and they were quick playing. So, at Historicon I asked about them and if I could get a copy. I was surprised to find that they were a published medieval rule set called TACTICA, 1992, by Arty Conliffe. Since the con I have ordered and received the rules which are now out of production. But used copies abound.
What I like is that they use a d6 for very simple and quick resolutions. Missile casualties are determined by a ratio of shooter to troop quality and cover. Melee is opposed rolls.
The basic sequence of play is: 1. Dice for move option 2. Movement- player A then player B 3. Missile fire 4. Melee
The scale is 1 figure= 80 men; one inch= 15 yards; one turn= 15-30 minutes. Suggested figure scale is 25mm or 15mm.
If you are considering running any mass combat that you might otherwise use Chainmail for, these rules are worth a look. I would recommend them if you are considering a game involving a siege. Complete rules for such a game fit on one page- rams, catapults, mining, ladders, rocks & boiling oil, towers, bombards.
Downside is similar to Chainmail, such games require a considerable miniature collection.