@Gary
Chill out. I destroyed your stack against all odds with one veteran Archer. I didn't see you Siege Tower first, but attacked your stack, showing a mangonel. The Siege Tower took the defence instead. Your Siege Tower was not fortified, and on undefended terrain, and while still a strongpoint - amazingly though, it took the blow.
If the Siege Tower had been on fortified grounds, or better a more defensive terrain, my Archers wouldn't have had a chance, veteran or not.
When I designed the units for this scenario, I did a tremendous amount of research on medieval warfare. I wanted a balance, that made missile fire important, Archers and Siege Engines in particular, as sieges were paramount in medieval warfare. I wanted Siege Towers to be a temporary base of defense, allowing one to bring in a buildup of mangonels, archers, defensive and construction units, outside a city.
I didn't want, however, that this should be impregnable, as it would be completely unreasonable in you could completely rely on just a Siege Tower for the defense of a complete area (not to mention the AI stacking them in their cities for defenses, which I wanted to avoid). So I made sure that missile units had a chance (if slim, in the case of Archers) of damaging a Siege Tower.
This would be quite common in a siege situation. Archers on the city walls attacking the approaching enemy, and the Archers on the Siege Tower returned fire. In many cases you need to bring in the monks or craftsmen and build a castle on the site as a more permanent base for an attack.
On flat ground, even mounted units will have a shot at cracking a Siege Tower, if they are able to attack it. This is also historically grounded, as mounted units in a besieged town or castle often would attempt a breakout from the siege.
Another consideration I made, and which is of paramount importance in this scenario, is that terrain is everything. This is also historically grounded. Every unit in this game will be likely to lose to a just slightly able attacker on undefended grassland. And even the weeniest spearman will be mighty, if fortified on a mountain site, within a castle.
The way to think strategy in this scenario, IMO (although I've seen the Duke employ other unusual interesting defensive measures for the Scots) is in terms of terrain, and control of areas. Abbeys are deliberately extremely rich, and with no defensive bonus (i.e. equivalent to grassland), making them the ultimate target of warfare, and difficult to protect. Most cities are also sited on grassland, with some notable exceptions, and are often very difficult to protect, unless walled.
If you are in striking distance from a city or area, from a well-defended Castle placed on a hill, riverside or mountain, the enemy has a very difficult time in conquering your cities and Abbeys.
Civ actually reflects the latest historical research in medieval fortifications quite well. This emphasizes construction of castles as a means of expansion, of controlling conquered areas, rather than, as traditionally thought, defensive strongpoints and sitting ducks for a siege.
In this particular case, Liège is quite well protected, sited on a hill by the river, and your inability to gain a strongpoint outside the city. Unless you can get into the Frankish occupied hill southeast of the city. As for your spearmen etc, they were hopelessly defending on flat land, against my superior mangonels and king.
In the latest update of this scenario, I've made Archers a wee bit more expensive, as I long have considered them the far best unit of the early game.
All that said, my veteran Archers beating your Siege Tower singlehanded is still a rare situation. They were almost destroyed, barely surviving.
