As the thread stated. When will they appear? Is it after a set nr of shards played? After a set number of shards beaten? When you take a certain lvl of building? Random?

The reason I'm asking is this... When you choose shards you should always try and go for the shards that will give you the buildings that you think you're going to use. I've done that and gotten my first set of lvl 2 units now and my first real enemy has arrived (not counting the helpful one here). Now I have some shards with low lvl buildings that I don't really need, but they have some bonuses on them that could be useful. If I take those small bonuses will the opponents get stronger benefits or will they stay with me in strength (strength in buildings that is)?

AFAIK, the turn in which you meet each of the other masters is fixed. The first half-dozen or so show up about every 5th shard. Some of them may be more friendly than others, depending on their outlook and your karma. The main thing to watch out for is that you'll be invaded by the third one shortly after meeting him, and losing your home shard defense will end the campaign.

Thx for the info.

But what about the second part. Do the enemies have access to more buildings than I do or similar?

I'm not certain. I've seen it claimed both ways (AI progresses based on yours; AI progresses based on astral turn.)

You probably won't run into any serious trouble if you grab an occasional old shard that has something you really want, although older buildings that you don't yet have may eventually spawn in new shards.