Game exploit things to know: 1) When you "Buy Down" in the vessels page, you are refunded the materials it would cost to get your current vessel. 2) the materials used to get a "Medieval Tank" cost more than the materials to get a "Vanguard".
If you don't like that exploit because it's a little too gamebreaking (you can go from 15,000 gold to 500,000 in a few minutes), you can do a variation of it: Before upgrading your vessel, buy down to "On Foot" to refund the materials you already invested in your current vessel. This lets you upgrade at a discount, without breaking the game.
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