I think that idea has merit from a standpoint of ease of managing your traide,s mq, but it makes it hard to think about how you could hold trade routes at risk in a war situation (or even from the creatures of the wild). Could you lay out a mechanic that allows for trade route disruption in the "trade screen" setup?
Maybe we reimagine the whole concept of trade.
Here's a thought: Each level of city earns a number of "trade points". Let's say one per level. Let's say you have 2 level 5 cities, A and B, and 5 level one cities, V, W, X, Y, and Z. A and B each have 5 trade points. V-Z each have one. You can spend one trade point in each city to build a trade network between them, using a trade screen as you have specified above. When you do that, a road automatically appears between those two cities that grant each unit moving on it +1 moves. There are 5 levels of road: Path, dirt, stone, paved, and magical, and each level confers a +1 move bonus to each unit on it. Each level of road also confers a trade bonus; +5%, +10%, +20%, +40%, +100% (or some such with balancing). You "level up" a road by choosing to spend more trade points between two cities. So, I could spend all of A and B city's trade points in establishing a magical road between them, but then I have no path to V-Z (V could have a path to W, say, and X could have a path to Y). Or, I can make a dirt path between A and each of V-Z, getting less trade bonus (A would get 5x5%=25%, V-Z would each get 5%, for a total of 50%, as opposed to A and B getting 100%, and V-Y getting 5%), but letting me move troops a little faster to each of those (likely to be frontier) cities (or outposts). The reason I like this mechanic is it provides a good "guns vs. butter" choice for me: better trade, or a little better troop movement and defensibility of my outlying realm.
Roads can be attacked by a simple "disrupt" action (it should be a trait, not every army can disrupt a road, just those who are designed to do so, like engineers). If you disrupt a road, the trade link is lost, each of the cities on the ends get back their trade point (ie, it's not gone for good), but you have to set the trade route up again. Let's say it takes 1 turn/level to set up the roads (ie, magical roads take 5 turns to set up, growing each turn from path through to magical).
Okay, that's an alternative idea that abstracts trade from caravans being a unit, needs no builder unit, allows for strategic choice, and can still be held at risk as an asset, which allows for economic warfare.
Winni
