Siege Strategy: Where to Build and How the Waves Scale

Updated October 2026

Siege is a compact tower defence game: one winding road, one kind of tower and one kind of invader, with every wave a little larger and tougher than the last. With so few moving parts, the whole game comes down to two decisions you make over and over: which green square gets the next tower, and when you can afford it. This guide gives the real numbers behind both, a map of the strongest squares, and a few habits that carry a run as far as the waves allow.

The rules in numbers

The board is nine squares wide and thirteen tall. The brown road enters at the square marked IN near the top left, snakes back and forth across the board four times, and leaves at the square marked END near the bottom left. It is 37 squares long, which leaves 80 green squares for building. You begin with 150 coins and 5 lives, and every invader that reaches the end costs one life. At zero lives the run is over.

Towers are not a flat price. The first costs 50 coins and each one after that costs 25 more than the last, so the second is 75, the third 100, the tenth 275 and the twentieth 525. The price of your next tower is always shown in the middle of the top bar, in green when you can afford it. Your starting money buys exactly two towers with 25 coins left over.

Every tower behaves the same way. It looks for the nearest invader within 2.4 squares of its centre, turns toward it and fires a shot every 0.55 seconds. Each shot does one point of damage. Shots fly in a straight line toward where the target was when the tower fired, so they do not curve, and a fast target can occasionally slip past one.

Defeating an invader pays 10 coins and 10 points. Clearing a wave pays a bonus of 50 times the wave number, in coins and in points alike: 50 after wave 1, 500 after wave 10, 2,500 after wave 50.

How each wave gets harder

Wave n sends 5 + n invaders, so wave 1 has six and wave 20 has twenty-five. Four things scale:

• Health is 1 for waves 1 to 3, 2 for waves 4 and 5, 3 for waves 6 to 8, 4 for waves 9 and 10, and then rises by roughly one point every two and a half waves. From wave 11 that base figure is also multiplied by 1.03 for every wave past 10, so the extra compounds: about 11 at wave 20, 22 at wave 30 and 39 at wave 40, with no ceiling.
• Speed starts just above one square per second and grows by 0.06 squares per second each wave until it reaches 2.8 at wave 30.
• Spacing between invaders starts at about 0.7 seconds and shrinks until it bottoms out at 0.35 seconds from wave 18.
• Count grows by one per wave, forever.

The practical takeaway is that the early game is about health and the late game is about volume. After wave 30, the invaders stop speeding up, but their health and numbers keep climbing.

Build before you send the wave

Between waves, the board shows a banner inviting you to tap the road to start. During that pause, tapping a green square you can afford builds a tower there and does not start anything; tapping the road, a square that already has a tower, or a square you cannot afford yet sends the wave. So place your opening two towers before wave 1 begins, then tap the road. Do the same before every later wave: spend first, start second.

Inside a wave the rule is the same. Money earned during a wave should be spent during that wave, the moment the price in the top bar turns green. Coins sitting in your purse do nothing, and the bonus at the end of the wave will arrive whether you spend now or not.

A map of the strongest squares

Because the road zig-zags, many green squares sit between two lanes and can reach both. We counted how many road squares fall inside a tower's 2.4-square reach from every green square. The pattern is very clear:

• The green strip between the first and second lanes (the single row just below the top lane) is the best area on the board. Its middle squares reach 10 or 11 road squares each, because they cover the top lane, the second lane and the bend on the right.
• The two rows inside the second loop (between the second and third lanes) reach 8 to 10 road squares, highest toward the left side near the vertical stretch.
• The two rows between the third and fourth lanes reach 8 to 10, highest near the right-hand bend.
• The top row, the far right column and the bottom are weak. The top row and right column reach only 3 to 5 road squares, and the bottom-right corner reaches none at all.

How much does placement matter? In our simulation, a player limited to ten towers who always built on the highest-coverage square survived to wave 17, while one who filled the board from the bottom up fell at wave 11. With twenty towers, the gap was wave 25 against wave 17. The same money goes half as far again on good squares.

Why nearest-first targeting shapes your layout

Towers shoot the closest invader, not the one furthest along the road. A tower beside a bend often ends up shooting fresh arrivals while an earlier invader slips away on the other side of its reach. Two ideas follow from that. First, spread your early towers along the road so that every stretch is covered at least once, then double up. Second, protect the last stretch. Once the late waves arrive with high health, the invaders that survive the upper lanes all funnel down the left-hand column to the exit. The squares just to the right of that final column, in the two rows below the fourth lane, are your last line and are worth filling before the weak edges.

How a run ends

Every run hits a wall, and placement decides where. Because health compounds from wave 11 while the board only has 80 squares, there is a hard limit: in our simulation even a board given all 80 towers for free before wave 1 fell at wave 44. A player who bought a tower the instant it became affordable and always took the highest-coverage square lost no lives until about wave 39 and was overrun at wave 41, after roughly 11 minutes, with about 51,000 points and 62 towers built. Filling squares in random order ended between waves 36 and 39 (about 40,000 to 46,000 points), and filling from the bottom up ended at wave 36 (about 40,000 points). Those numbers were the same at every screen size we tried.

That has two practical lessons. Fill the strong squares first, because by the end towers cost well over a thousand coins each and you will never own the whole board. And treat the last few waves as the real contest: the bonus for clearing wave N is 50×N, so each wave you survive near the wall is worth more than the one before, and that is where good runs pull ahead.

Common mistakes

• Saving up. Tower prices only rise with the number you own, never with time, so waiting buys nothing.
• Building on the weak edges first. The top row, the right column and the bottom-right corner should be the last squares you fill.
• Leaving the exit column thin. One escaped invader per wave adds up quickly with only five lives.
• Starting the wave by accident. Between waves, a tap on the road or on a square you cannot afford yet sends the next wave. Check the price in the top bar, build, and only then tap the road.

Your best score is saved on your device, and any score above zero can be sent to the Hall of Fame from the end-of-run card.