Tue Apr 08, 2014 8:22 am
It sounds like Gettysburg did not have any Union troops in the structure, whereas Norfolk and Morgantown did. If there are no enemy units present, you automatically capture a city when a line element (not a Cav or Irreg) in Offensive (Orange) or Assault (Red) posture enters the region.
When there are enemy units present there are two cases, they can either be in the structure (in this case the city) or in the region. If they are in the region you will usually see their unit icon in the region if you have good enough detection; you may or may not be able to tell exactly what is there, hover over the region to see the info you have available, being sure to look at the bottom of the tooltip for "Also Here" units. If they are in the structure, their map icon does not show, but you will see a number of green dots next to the city in a little box, and again, if detection is high enough, hovering over the structure (or maybe the region, just hover over everything and look at what it tells you) may tell you more precise info about them if you have good enough detection.
If they are in the region, you enter in Orange, Offensive, posture (the top button is Posture, the bottom button is Rules of Engagement) and a battle will ensue or they will retreat before battle starts. If you win the battle (the enemy retreats from the region) then you capture the city unless they also have troops inside the city. You only engage troops inside the city if you are in Red (Assault) posture.
If they are in the structure when you enter the region in Orange posture, you will not engage them, but will initiate a siege, and the golden mortar icon appears showing that they are besieged. Once the icon is on the map you can switch to Blue (Defensive) posture while still maintaining the siege. Besieged structures do not produce anything, cannot receive supplies from the outside world, and cannot receive replacements for the elements inside. If the region has a harbor then they can still receive replacements and draw supply through the (water based) supply network(s), but do not produce on their own (I might be wrong about producing, but they definitely get supplies and replacements). To cut off supply and replacement you must blockade the harbor. Hover over the harbor, and it will say in the tooltip which water region the harbor is connected to. Hover over that water region to find out how many naval combat elements are required to establish a blockade.
Each turn you besiege, the engine will make a random rolls for each side modified primarily by artillery power. If your modified roll is higher than the defenders by a certain amount you succeed in breaching the fortifications, and they no longer receive a defensive combat bonus from the structure. Sometimes they will take hits. I think but am not sure that if you make a second breach they do not get their entrenchments either, but not 100% on that. I think it is a d10 roll. You get plus one for every ~30 power of artillery you have, and some leaders and units have abilities than give bonuses to the siege roll.
If you have considerably more troops besieging than the defenders have there is a chance that they will surrender without a fight. If they do, you typically win VPs and NM and do not suffer any losses. This can take several turns, depending on force sizes and other factors. They will very rarely surrender even when outnumbered if they have a wagon with supplies in the structure. So if they have an unblockaded harbor and a wagon they will probably never surrender because the wagon will keep getting supplied through the harbor. You can try to starve them until they are weak, but many cities have large supply stockpiles which may make starving them into submission practically impossible even if there isn't a harbor or you have it blockaded. Depots are especially problematic, as they usually have very large supply stockpiles, and a besieged depot with an unblockaded harbor allows the defenders to receive replacements at the maximum rate.
In order to engage units inside a structure directly, set your posture to Red (Assault). This is basically the same thing as Orange (Offensive) posture, but you have specifically given leave for your forces to attack enemies inside structures. You will fight a battle, and if you destroy the enemies then you capture the city. They cannot retreat from your assault, so remain stuck in their structure even if you do not wipe them out in one go. They get a lot of combat advantages, so expect higher casualties than normal when assaulting. A good rule of thumb is that you need three times their power to be sure of winning. If you besieged them for a while and made a breach or they run out of supplies then they will be weaker when you assault them. (If they have run out of supplies you might want to hold off the assault and see if they will surrender first to minimize casualties).