(Note that there are 4 Simple Keys in the game for 4 Simple Locks, with one key required for 112% and one key recommended for easy access to an area, so choose your key usages wisely.
In some cases, your Shade will spawn in a poor location that makes retrieving it hard (e.g. In early game when you don't have a surplus and don't have banked Geo in the form of relics, Geo is fairly important so losing all the collected Geo may force you to farm enemies.
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The single largest "required" purchase for beating the game (if you don't want to navigate dark rooms blind) is the Lumafly Lantern from Sly at 1800 Geo while the single largest required cost for 112% is the Pure Nail upgrade at 4000 Geo (upgrading Fragile charms to Unbreakable is not required). Nevertheless, important sources to consider for geo early game include:
While you can farm enemies for geo, further areas in the gradually scale up the amount of geo dropped from common enemies so if you're patient there isn't a need to farm. Geo in early game is tight, but by end-game you will have too much At the start of the game you'll find yourself fairly short on the geo amount you want to have in order to afford several useful items from Sly and Salubra.