A catapult has a bomard of 4. A fortified spear in a flatland town with walls or a city has a 10% bonus for flatland, 25% for fortification, and 50% for the walls/city bonus. So, the spear effectively has a defense of 2+.2+.5+1=3.7. Without walls it's 2.7, and the AIs might not have walls or a size 7 city in the ancient age. Compared with trebuchets vs. pikes you have a bombard of 6 vs. 3+.3+.75+1.5=5.55, or 4.05 which comes as less like than in the ancient age. So, trebuchets only have a .15 advantage over catapults if both cities have walls. With cannons vs. muskets we have 8 vs. 4+.4+1+2=7.4. Artillery vs. rifles 12 vs. 6+.6+1.5+3=11.5. Wait... the civilopedia says that walls provide a land bombardment defense of 8??? Did they make an error in the civliopedia? If not, do cities (i. e. locations size 7-12) still have that land bombardment bonus? Or does that defense bonus of 8 for walls not come in addition to the other defenses of the unit?