Developed by Moi Rai Games and published by Team 17 in December 2020, Monster Sanctuary is a creature collector 2D platformer RPG that a friend of mine was so enthusiastic upon playing that he bought a copy for all of his favorite people for Christmas in 2021 - rather rare for a single player game. As maybe a bit less of an enthusiast, it took me a few years to get started on it enough to really get into it, but once I did, I could certainly see the appeal. The game has a lot of complexity in combat and people who enjoy games centered on building teams will find plenty of options to work and play around with.
Players who are less into building teams (I may be guilty of this) might have a bit more difficulty getting into it, but there's still a lot to enjoy as long as you like exploring high-quality pixel worlds and gathering animal friends while listening to an original soundtrack that gives great retro JRPG vibes. It also helps that there are plenty of external community resources that can help newer players find good builds to aim for when needed.

Collecting a Monster Team
In Monster Sanctuary, you play as a Monster Keeper from a family that has a monster Familiar tied to it which is passed down from generation to generation. With this familiar (that the player can select from a choice of four options) now passed down to you, you begin the game as a Keeper who is just starting out, with your familiar as your only monster.
As expected of a creature collection game, your familiar won't be your only monster for long. Wild monsters are dangerous and you can't befriend them, but if you fight and defeat a monster, there's a chance it will drop an egg of its monster type. Hatching this egg makes the resulting monster loyal to you and you can train it to fight other monsters and repeat the cycle.
Monsters on your team aren't only good for combat. The world of Monster Sanctuary involves exploring a 2D platformer world with various obstacles scattered across the map. Different monsters have different map abilities, so hatching a monster with an ability you can use to get past specific blocks is crucial for progression.

Battling with Monsters
There are two main combat types encountered when wandering the sanctuary: Monster Battles and Keeper Duels.
When traversing the map, you set a team of six monsters that will fight any battles you encounter. If encountering a monster battle (the monsters you meet on the map), you choose three of those six monsters to fight as you enter the battle, having access only to those three for its duration.
Keeper Duels are a bit different because you fight with your full team of six but still only have three monsters active at a time. The winner is determined once a keeper's entire team is defeated. This is the form of combat used against opponent NPCs as well as in the PvP arena.
The battles themselves can be rather complex. Monsters have elements, ability trees, gear, and other aspects that affect their individual builds and how they can be used in combat. There are buffs, debuffs, combos, and other things that stack during combat to have huge effects on outcomes. For casual players who don't delve into the system and experiment with builds, this can make combat daunting past the starting areas because you can't just level up your monsters and expect them to win; you need to create teams with strong synergy. Fortunately, there's a lot of leeway for experimentation since new eggs hatch at close to the level of your highest monster and there are items you can acquire to level monsters further and avoid grinding.

Fighting on Past the End
If you're just in it to unlock the map and complete the story, there isn't much to get out of replaying the game. Especially since the map doesn't change and the story itself is pretty light - it's excellent at giving a reason behind exploration and combat, but lacks any special draw, serving more as a support for the core aspect of the game, which is its teambuilding mechanic.
However, there is a good deal of replayablity for those who love that core aspect of teambuilding. There's a New Game+ mode which allows you to start over, keeping your monsters and equipment. There are also settings that offer randomized monster locations, permadeath, "relic" equipment for wilder play, and a mode with limited randomized monster recruitment so you can challenge yourself to create a strong team from a small pool of options.
Quick Reference
Monster Sanctuary is a 2D platformer creature collector RPG with a focus on building teams.
Best played with an enthusiasm for delving into complex systems.
Strengths:
• Huge variety in team-building.
• Deep tactics in combat.
• Quality art and music.
Weaknesses:
• Combat can be daunting for casual players.
• Though it shouldn't be an issue, the game can still feel grindy with poor planning.