Buildings and Wonders
You will recognize many of the buildings from the un-modded game, but here are answers to important questions you might have.
How do I get culture to pop my borders? Religious buildings are your best source of culture early -- there is no early monument. If you don't have a religion spread to you your best options are to hold off researching Theology (so you can build Pagan Shrines) or to build Manor Houses.
How do I keep my cities healthy? Health was a problem for large medieval cities. The equivalent to the aqueduct, the Herbalist, gives only one health. There isn't a lot of health to go around, so you can either live with unhealthy cities or keep them small.
What's up with the market? The market is now a prerequisite for most buildings which sell goods. The buildings enabled by the market (e.g. butcher and grocer) will allow you to run merchants and generate commerce if you have the appropriate resources.
Why are there more religious buildings? Religion was such an important part of medieval life that we took several building features and re-assigned them to religious buildings. The library has been replaced by the scriptorium (for Catholic/Orthodox civs). The colosseum has been replaced by the belfry/minaret. Each religion now has a larger number of (different) buildings. Generally, if you want culture or science, you want to build religious buildings.
What else should I know about? Archery ranges function like stables but for archery units. Barracks, stables, and archery ranges cause unhappiness, so build them carefully.