Info taken from FAQs in file area and various forum threads, consolidated into a Wiki page so that people can more easily find all the info in one place and update it.
After Round 1, each player will have 5 face-up cards played. After Round 2, each player will have 6 face-up cards played (1 of which is the relic they kept from Round 1). After Round 3, each player will have 7 face-up cards played (2 of which are relics they kept from Round 1 and Round 2).
After Round 1 & Round 2, players simultaneously keep (as a "Relic of the Past") one of the 5 cards they played that round and discard one of those 5 cards. Thus they retain 3 of the 5 cards they played that round in their hand, and add 2 more randomly drawn cards to those, to start the next round with 5 cards in their hand.
Do not trade hands or shuffle cards between rounds.
05 The Mana Well (Studnia Mocy)
You cannot use the same instance of a suit in more than one set when scoring this kind of "points for each set" card. E.g. if you have 2 crowns, 2 towers, and 1 hand, then you have a single set, not 2 sets. (Same for Sky Pillars.)
09 The Maze of the Damned (Labirynt Potępionych)
If you have a set of 5 symbols, then you get 13 points. You do not get 13 points for each set! (You could have 2 sets thanks to Roof of the World...) The wording on the card is intentionally different from Mana Well and Sky Pillars!
14 The Sky Pillars (Filary Niebios)
You cannot use the same instance of a suit in more than one set when scoring this kind of "points for each set" card. E.g. if you have 2 scrolls and 1 plant, then you have a single set, not 2 sets. (Same for Mana Well.)
16 The Molehill (Kretowisko)
Each player counts their singleton cards, i.e. how many suits they have represented by exactly 1 card. If your count is higher, then you earn 8 VPs. Note that Molehill intentionally refers to your number of "cards", not "suits". So if you have 1 card of each suit and you also have Roof of the World, then you have 2 of each suit, but you have 5 singleton cards for purposes of Molehill.
17 The Roof of the World (Dach Świata)
It means what it says! So if you have exactly 1 card of each of the 5 suits, then all 5 of your suits are "your most numerous suits", so it doubles all 5 of your suits, thus you (in effect) have 2 of each suit. (But you still have only 1 card of each suit, for purposes of Molehill.)
The doubled number of suits is used for all purposes in scoring (e.g. the 5 cards giving 3 points per symbol, and the 5 cards giving 7 points for a majority, and the 3 cards giving points for sets of several suits.)