Combat on Buildings terrain

Usually a city next to buildings terrain will be sieged first, since sieges are even easier from buildings than from mountains (unless you are sieging a dwarf with a lot of dwarf friends nearby).  Ranged units have a bonus in buildings, but not as much as defending spears or defending or attacking infantry.

Looking at the chart, being sieged from a mountain looks like a bigger problem, if the sieging army uses all or mostly ranged units with the mountain defensive bonus. Perhaps a 100% ranged unit sieging army may be harder to assemble?

A related question: Can many players form one army to place on a sieging square, or can only one player occupy a sieging square, limiting the number of players that can siege a city to 8?

Hiya Whistlemist. Honestly, I really don't think that's the case. In fact, when it comes to being sieged, I'd say that being in an alliance is actually more likely to get you sieged, since if the alliance you are in gets into a war, your city might be chosen simply by virtue of the fact that it sits next to terrain that is favorable to defend (alla buildings or mountains). This isn't to say that non-affiliated players never get sieged, but my experience tells me that those who are sieged are usually gc trolls of the worst kind. If you're just minding your own business, you're most likely not going to wake up one day and see a siege army rolling into your back yard.

Also, regarding building your city next to buildings, I wouldn't do it. Yeah, they look cool, but as you can see from the chart above, it's no secret that they afford optimal terrain bonuses to bows and infantry. Recently in our current war we sieged from a building and had fabulous kill ratios in defending the siege. A best practice is to make sure your city is surrounded by plains, since cavalry work optimally on plains and can really chop down a siege.

Best of luck to you in your endeavors.