To enter the Harem Route you must pick "I like ais..." at the beginning, unlock the brothel, do all 6 scenes there before Ishtar's saga and do not get in any relationship (casual stuff with routeless characters like Demeter is fine), then during Ishtar's saga you will have the chance to stand your ground against Aisha, defeat her in battle, and you'll be entering harem route. It includes heroines like Hestia and Liliruca, but that won't unlock their single route scenes... They get entirely new scenes. So by all means, the harem route is just another route, but with multiple heroines.
The routes for this game are the following:
-Hestia
-Eina
-Liliruca
-Harem (Lena, Samira, Aisha, Hestia, Liliruca, Haruhime)
-Haruhime
-Airmid
-Cassandra
-Artemis
There are also routeless nsfw scenes, mainly with Demeter. Some require playing some side quests, whereas others are unlocked through achievement scenes.
It doesn't matter if you like Ais or not, by picking "I like Ais" at the beginning you take the common route where the Brothel location is unlocked. It doesn't get unlocked right away since Bell won't just wake up a morning and be like "Let's slay some Amazons with my secondary dagger!", but it'll get unlocked around the time he gets his first spell and learns about Mind Down status, so much later in the story.