What are the bonuses and penalties for different combat units when fighting on terrain described as [Buildings]? Which are the best units for [Buildings] combat? I'm thinking that my general military strategy, which is evolving around combat in mountains, could be undone by this square (all other squares around it are mountains and hills).

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.

*Terrain bonuses attached to the bottom

Of note is the fact that despite the terrain description, ranged units receive a substantial bonus when attacking on building terrain.   

Attack:  Sword > bow > spear > cavalry

Defense:  Spear > bow > infantry > cavalry

Illyriad is a game of rock/paper/scissors in the sense that while there might technically be a best unit for a terrain, there is an equally best unit to counter that specific unit.  So,  despite the above surface observation the biggest factor will be what is the make up of the forces you will be combating.   

vs. spear defense:  Attack with bows // 

vs. bow defense: Attack with cavalry // 

vs. a combined defense: Attack with Swords //

I think a few alliances might have it as well or a link to where it can be found.  

I would suggest this http://www.puzzleslogic.com/illy/home.html" rel="nofollow - site as well for some good charts/information regarding Illyriad basics. 

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?

Typically a serious siege will consist of several blockades and one or more sieges.

Honestly, a city surrounded by mountains and a building would be a dream to siege.  Having it be on a mountain would be a negative.  It would be possible to defend such a city, but I would anticipate having heavy losses on both sides.

I think there might have been a city like that in the last war -- Praetor Augustus's city maybe?  The city did not fall, but a lot of troops died on both sides.  I was not personally involved in that one though, so someone else would have to confirm my recollection about whether it was a mountain surrounded by mountains.

By the way, in that situation as a sieger I'd make my decision about whether to siege from mountains or buildings based on my opponent.  If it were an elf, I'd put something in the buildings square, probably defended by a mix of spears, bows and t1 infantry.  If it were a dwarf, I'd avoid the buildings and stick everything on the mountains.  Of course, the dwarf could have fooled me by building crossbows.  It is helpful to know the likely makeup of the armies of nearby defenders in these situations.

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.

Defense against sieges is one factor to consider when placing a city, but there are a lot of other ones as well.  Most players go through years in Illy without being sieged.

I don't recall a situation in which nCrow has sieged a player for settling in the wrong place.  In fact, nCrow has a long track record of coexisting peacefully with neighbors.

There was one situation in which a leader of nCrow rashly sent a siege to a player who had settled on a square that he had marked with a nearby army indicating he intended to settle there.  The sieges were recalled, that leader was demoted and left nCrow.  We apologized to the player, who apparently did not hold the situation against us because about a year later he decided to join our alliance.

I do recall a player, Malpherion, who left VIC and joined Harmless? because he was in a dispute with a player over cities that had been settled near him.  VIC would not allow him to siege the player over a settlement dispute, but Harmless? approved those sieges.

So in terms of sieging players because one has decided they settled in the wrong place, I think KillerPoodle may be confused in his characterization of which alliances do this.

In that case ...