Feels tricky at first, but once you realise the key rules it's quite easy.

The thing that's not immediately obvious is that while adding each of the components of the tower uses up a spanner, the "repair kit" can be used whenever you have a spanner without using the spanner up.

So as early in the game as possible just pick up all four spanners (easy when you know where they are!) and you can keep nipping back and forth to the vending machine to buy ammo / health kit for you / repair kit for tower, and keep doing this indefinitely until your cash builds up. Whenever you leave the tower and return to it, it's usually crowded with the Undead, but flamethrower that big mass of them and and you'll have time to repair the tower (and add any components you bought) while fending newcomers off with the machine-gun. When ammo is running a bit low and you've made more cash from the fighting, head back to the vending machine and repeat. Helps to blow up some crates while you're on your way up, to build up cash faster.

Game is fun and controls pretty smooth, would have been funner if the game mechanics had been explained earlier (the tutorial is nice but how the component/repair/spanner system works is not made clear).