"We go through the city gates and head for the.."
"Not so fast! All entering the city today must pay the:
1. Wheel Tax - 1 silver for every wheel on any cart, wagon, carriage, etc.
2. Sword Tax - 1 silver for every bladed weapon larger than a dagger, yes that axe too, mister.
3. Purple Tax - 1 gold for every purple article of clothing. "Show me the waistband!"
4. Sewer Tax - 5 copper for every adult, though I don't mind the smell myself.
5. Road Tax - 2 copper per person for road repairs, 1 silver per horse or mule.
6. Holiday Tax - 1 silver per adult, the Feast of Barnabus nears.
7. Bag Tax - 3 gold per bag of spoils carried, and you're lucky I don't look inside.
8. Staff Tax - 5 gold per magic item carried, scan em' Dan'o.
9. War Tax - 1 silver per person. Troops are mustering and need supplies.
10. School Tax - 5 copper per person, for the new university.
11. Shield Tax - 5 silver for each person or mount in medium or heavy armor.
12. Fey Tax - 4 silver for each non-human, mounts and livestock excluded.
13. Library Tax - 4 gold for each book carried.
14. Brass Tax - 3 gold for each clockwork device.
15. Spoils Tax - 1 silver for every pound of treasure carried, on the scale buddy.
16. Bow Tax - 1 silver for every bow or crossbow, and unstring it.
17. Minstrel Tax - 2 gold for every musical instrument.
18. Crown Tax - 1 silver per adult, the King's visit must be grand.
19. Holy Tax - 1 gold for each holy symbol not of our city's deity, heathen.
20. Tithe - 5% of all coin carried, for the new cathedral.
These may either be in lieu of or in addition to the usual gate fee to get into the city. They in no way obviate the standard taxes for any sort of trade goods brought into the city. Trade goods are always taxed, regardless of whether the bearer claims intention to sell them or not. Remember that if your dwarven friend likes to carry around a barrel of stout in your cart.
Big merchant caravans will usually camp well outside the city, protected by outriders and goons.. er, guards, to avoid paying taxes in every city they might need to pass or sleep in. If they start setting up too close, perhaps 100 yards or so, the guard will send a detachment to politely but firmly run them off. To enjoy the city's protection, you have to pay her taxes.