The Hospital is the most essential construction, for the sake of keeping Merim and Milli alive. If you build the hospital first, it'll be finished about the time you obtain Merim via optimal schedule, so you can make the most utilization of her.
The second important construction is Maria's research building. You need to talk to Maria once, attempt to H Maria once, and talk to Maria once more for it to show up to be built. When it is completed, visiting it 5 times will cause the attack power of Maria's troops to increase to 150%. Visiting it 4 more times (9 total) will lead to the completion of a great cannon that will be located in the town of North. You may spend a city event to set it's target, then every week for the first battle at that location, the cannon will eliminated half of the troops of the first unit. This helps greatly offensively and defensively. After 3 more visits (12 total) the research building will explode, and Maria will die if the Hospital isn't built. If there is a Hospital, Maria will temporarily be hospitalized. When she recover, there will be a new CG when you H her. On either the 7th and 11th visit picking the top choice will lead to Maria's happy end.
The third important construction is the Girl Officer Academy. It will produce two fairly decent captains after a few visits and a few weeks of waiting.
Of arguable importance is the Magic Research Building. After its completion, you may ask the researcher to power up Asuka, Melfeis, or Shizuka's magic power. The time required to increase magic by 1 is twice the current magic level. For example, Shizuka's initial magic is at 8, thus it will take 16 weeks of research to reach 9. It takes a long time, so it pays to start early. On the otherhand, the increase might be considered too small for such a long time, so whether it is worth the efford depends on the player. Note that Asuka and Shizuka both have "bombing" skills, so boosting their magic power can potentially make a big difference against massive enemy forces. It is also associated with Papaya's happy ending, and needs to be completed at least a month after defeating Papaiya for the happy end.
The Library is associated with one H scene and the acquisition of Bound. Bound isn't a strong character overall, so there might not be any hurry to get him if you won't be using him much. On the other hand, getting him early can allow him to accumulate a few levels to be useful later on.
The SM tower is of the least importance. It's only for H scenes and character endings, and can definitely wait til the very end.
Lia and Maris have their happy and unhappy ends linked. To get their happy end beat the creator phase while not having Sill in your army, to get their unhappy end beat the creator phase while having Sill in your army.
Talking to Leila in Captains mode will make her show up in the Harem. Trying to H her will cause you to be rejected, but then the Royal Guard will now appear in the Harem. After H-ing the Royal Guard, talk to Leila in Captains mode, and now you can H Leila. To get her unhappy end, let Lia handle the negotiations with JAPAN (bottom choice, see JAPAN section). She will be traded to JAPAN. To get her happy end, after you are able to H her, let her fight alongside Rick for 4 battles. Then, wait for a start-of-the-turn event where Rick confesses to Maris but gets rejected. After that, let Rick and Leila fight together again to trigger their event.
If Leila is given to JAPAN, or she dies, Julia will take her spot as a commander. Julia's happy end is caused by taking her to the Hanny King and having him power her up, then seeing her start of turn events afterwards. The Hanny King is in the lake dungeon north of Lazeal, which you can only enter if you levy in Lazeal, equip a unit with the axe you find, take that unit to search the lake and do not pick the golden axe option. If you take Julia to the Hanny King, then take Rance and try to recruit him, you can give Julia to him as a pet. This will give you Julias unhappy end and get you the Hanny King as a commander. This will also cause Aviator to become the new Royal Guards commander and the Military Academy to close.
Talking to Menad will make her show up in the Harem. Trying to H her will cause you to be rejected. Afterwards talk to her twice, then let her fight in two battles where she shall retreat with less than 400 troops. Then on a random turn Maris will report something, after which point talking to Menad will lead to an event with a Black Army soldier, Menad, and Kanami. This is her happy end and will enable you to H her. If the last event with the Black Soldier doesn't happen, then she will die during the Creator phase of the game and get unhappy ending.
Asuka can learn her special skill after Mill has joined Leazas. Let Asuka and Mill fight alongside, and use Mill to attack at least once. Afterwards, a few random events (2?) will eventually lead Asuka to leave the army. A few more random events (4?) at the beginning of turns will bring Asuka back with her skill. It is unkown if the events in this chain are still available after Mill obtain an adult body.
Trying to H Kanami twice will provide you a choice of raping her during her period. After sending Kanami to spy on Zeth twice while you are at war with them will result in a happy end for Kanami and a game over. You must also need to rape Kanami during her period,order her to buy a burger and not forgive her as well as not forgiving her for being late otherwise she will protect Rance from being assassinated by Code and the game will continue as normal. Kanamis unhappy end is caused by having her get captured while on a mission.
After constructing the Hospital and talking to Aya at least twice, you will see a beginning of turn event where Aya rejects a soldier. Aya will then tell Rance that she would only have sex with him if he contracted Okayu disease. Take Rance to the 10F of natural foe dungeon where there is a purple Okayu Fever and choose to kiss it. Rance will contract Okayu disease from it. There will always be a purple okayu fever on the 10F so you can go up/go down if you pick the wrong choice.
Suzume and Wendy will randomly appear in the harem when you enter harem mode. You can get them both to appear on the first turn of Leazas phase by repeatedly entering and exiting the harem without choosing any girls. By default Suzume is in the unhappy ending. To get her happy end, H her twice, then randomly at the beginning of a turn you will see an event between Suzume and a Black Army soldier. Talk to Barres afterwards, and H Suzume again. Initially you cannot actually H Wendy, and she is in the unhappy ending. To be able to H her, you must get to the bottom of the Monster Cave and obtain the Big Boss's Pot. Then randomly at the beginning of a turn Wendy will break the pot, and you will be to use that as an excuse to H her. To get her happy end, you have to build the SM Tower, H her with it, and witness another random event involving Wendy.
Arlcoate and Rafalia will join the army from the Girl Officer Academy. The player must visit the academy to meet Rafalia first. Then when it's graduation time visit again to meet Arlcoate. The next visit will be the final exam, and the player will be prompted to choose between Arlcoate or Rafalia. Always pick Arlcoate first. Visiting the Academy again later will let Rafalia also join after 20 weeks or so. To obtain Arlcoate's happy end, talk to her 4 times after her joining you, before you H her. H-ing her early (before 4 talks) leads to her bad end). Rafalia will have her happy end simply be being in the army, whether you choose Arlcoate first and let her join later, or pick her first. Her bad end is if you pick Arlcoate first and never go back to the Academy so she never join the army.
Aviator is the teacher of Arlcoate and Rafalia in the academy. If you recruit both Arlcoate and Rafalia, she will get her happy end. If you lose both Leila and Julia as commanders, she will join your army and lock you out of recruiting Arlcoate and Rafalia, if you haven't already. If you then get her troop count below 200 in a battle, a start of turn event will trigger and you will get her unhappy end.
In order to get either of the ends for Kapala Lodz, you need to randomly get a scene with her while levying in Leazas (Oaks), then pick her event 'Nadatte TV' in Pretteria. To get her happy end you must never have sex with her and have her become the top idol, so have her sing for you and donate the maximum cash every time until you get a start of turn event where she's takes first place. For her unhappy end, you must have sex with her, get her to the number one position then take photos of her naked.
After building the library, if you have Maria, Kanami and Haurein, you will see events about a ghost in the library when you visit. If you keep visiting you will eventually have the option to add new books to the library. Pick Bishounen to get the Library Ghost's happy end and her H scene. Her unhappy end is obtained by starting her events but never adding Bishounen books to the library.
Continue to Rebellion