The AI kingdoms seemingly get "free" resources; but they don't use them intelligently. The AI's city layouts are very inefficient and quite hard to defend, so the vikings will eventually wipe them out due to sheer numbers.

When it comes to being attacked by AI kingdoms, they do tend to spam troops faster than the player can realistically compete with in terms of sheer numbers. However, once again they're not smart about it -- you can easily hold off the AI's attacks with some smart defences. The main thing to watch out for is that they'll often attack you right in the middle of a viking raid; but if you control the available landing zones (either by walling-off possible landing spots, or simply putting enough towers nearby to make other landing sites preferable) you can easily funnel all attackers into a kill-box. Note that to wall off a landing site doesn't just mean building walls along the shore; you have to build piers at the edge of the shallow water and then build walls there so that the walls go right up to the edge of deep water. Some strategically-placed towers are a very cheap way to handle initial defences; and a good trick is to build a tall ballista tower just behind an archer tower -- enemy soldiers and catapults attack the closest target; the ballista tower can out-range the catapults while they're busy with the archer tower.

Ultimately, the AI "cheats" so that it has a chance against the player; not to make it more powerful. It's a pain when they attack you out of nowhere, but their only real strategy is zerg rush + attrition; if their forces were strictly tied to their island economy they'd have no chance since the player can easily snipe their resource income. The AIs' armies are somewhat tied to the state of their islands/kingdoms though, so you can wear them down by destroying their troops and then forcing them to spend some of their income on rebuilding their other structures too.

When you first meet the AI kingdom you'll get an idea of their "personality", if they talk about being a powerful or prosperous kingdom that's your clue to start preparing for their eventual attack (they'll ask for tribute first; but the tributes grow over time so eventually you'll either have to refuse or you'll fail to meet the amount; and either way that leads to war eventually.)

Jo thanks, was suspicious about them cheating, because I attacked one very early (year 20) and burning their stuff down with a single unit. Still they built up again very quickly and thus remained resilient. In survival however, they eventually die due to viking attacks. Maybe in that gamemode they just get some ressources early game that exhaust over time.