Outline
There's a city (Small town). There's an ancient, magical tower next to it. Monsters are trapped inside. You are 16.
You can go into the tower, find treasures, weapons, armor, magic - and eggs. Take the eggs home, hatch them, have a new monster. Take the monster into the tower with you, level it up. Find "Sand", which can be used to level up your weapons.
Fight monsters, trip/avoid traps.
The character's level is reset each time he enters the tower, so he has to rely on his monsters to protect him, as well as his uber sword/shield.
Find "Special Items / Quest items" such as the Blue Collar, which allows you to use two monsters at once. Fuse the monsters, gain stats/abilities from both, as one creature (Albeit, you still have the appearance of the one creature

Once you return home from the tower, you can sell the treasures you found. You can spend the money on your house and monster hut, making them bigger. Or, spend it on improving the town. Casino. Temple. Racing Track. Library. Hospital. So forth.
At least 60 different monsters (That I can safely say. Mayhap 80+?). Considering the tower has 40 floors... Meh.
Get. Girls. I think it's 7 girlfriends you can have altogether, or was it 5? In the japanese version you can have a boyfriend, but they removed it from the US version (How... Gay.)
Eventually reach the top floor... And I'll leave that to you to find out about.
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There's probably alot more I left out, but that's some of my favourite parts (hurr). It's a Playstation One game from the 90s, in an Action-Turn-Based-RPG style. Available easily on Emulator, although it's probably against the rules for me to link that.
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Google is your friend.
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I've been playing this game for years, coming back to it every few months. It's awesome.