The first three towers decide everything

Most tower defence losses I see happen in the first minute, long before the boss wave. People place towers reactively, plugging holes as enemies leak through, and by wave ten the whole map is a mess of half-useful positions.

The fix is boring but it works: plan your first three placements around the longest path, not the nearest gap. A tower that covers a long stretch of road earns its cost many times over compared to one squeezed into a corner.

In games like Ironhold Siege this is even sharper because you can shape the path yourself. Force enemies into a single long corridor and suddenly two towers do the work of five. I usually spend my opening resources on lengthening the route before I buy any firepower.

Save upgrades for a moment when you actually understand the wave composition. Early over-investment in one tower leaves you brittle when the enemy mix shifts. Breadth first, depth later.