I'm inclined to think that medieval taxes would have been limited by the ability to manage the data.
Basically, we can do income taxes and sales taxes nowadays because everybody is required to report and track that. The accounting processes are robust enough to query how much you should pay and track that you paid it.
i suspect if you go back far enough in time, the processes and technology would have made it too cumbersome for such ideas to exist and therefore, simpler things to tax would have been defined.
A tax agency is going to prefer taxes on things that are easy to track and hard to be missed. Income taxes are easy to dodge in a primitive society, because everybody will lie about what they earned, and once the money is spent, it's hard to prove. Whereas property is sitting right there.
Things that don't move or change hands much (like actual land) would be good things to tax. It's not so cumbersome to have a list of all properties, their owners and their size and thus their value to be taxed. The taxman then just rides around to each area and checks off who pays and who doesn't which then earns them a personal visit.
ports and gateways are a good spot to tax incoming goods (tarrifs). If you're bringing in a barrel or more of anything, odds are good it's to sell it. So, it gets taxed when it arrives because its pretty easy to inspect the cargo and look at the tax sheet and charge a fee.
Governmental services are likely to be taxed, because as you use it, the money is routed straight to the government. So getting certain permits, using certain resources (like Imperial Portals). Roads might be taxable, if it really is a major road that actually solves a transit problem. odds are good, the locals will avoid the toll booth (getting on and off and the right spots or just using back roads). But the real value of major roads is for visitors (caravans) in using a more secured, better built road.
Citizenship might be taxable. It might be expected that each man have to pay X amount (maybe by trade). it stands to reason that each person be required to register with where they live (thus being on a list, so they can be verified to have paid the tax). This concept seems within the technology level, both the Bible refers to tracking of people (the reason Joseph and Mary traveled to Bethlehem) and in 1000AD or so, part of the big scare was the Domesday book, which was a big census undertaken for the purposes of taxes.
it would also stand to reason, if each citizen is registered to a town, that in order for them to relocate, they would need papers authorizing them (consider that if serfs are pseudo-property, the owner is transferring an asset). the request for such travel papers could be taxed.
You might also consider the organized crime "Protection" model. The king demands 1000GP from each lord for taxes each year. The Lords hit up their people with whatever excuse they want, to extract at least 1000GP from them (probably more, to cover their own annual expenses). This may or may not be levied as specific taxes, or just weekly protection payments from shop keepers. Thus shop keepers are motivated to make a certain minimum each week, so as to ensure they can pay their protection money.
