diff --git a/color.lua b/color.lua
index 033384b..551748d 100644
--- a/color.lua
+++ b/color.lua
@@ -4,9 +4,9 @@ COLORS = {
     TRANSPARENT         = vec4(0.6),
 
     -- tones
-    WHITE               = vec4(0.8, 0.8, 0.7, 1),
+    WHITE               = vec4(1, 1, 0.95, 1),
     PALE_SILVER         = vec4(193/255, 178/255, 171/255, 1),
-    BLACK               = vec4(0, 0, 0.02, 1),
+    BLACK               = vec4(0, 0, 0.05, 1),
     VERY_DARK_GRAY      = vec4(35/255, 35/255, 25/255, 1),
     TRUE_BLACK          = vec4(0, 0, 0, 1),
 
diff --git a/src/game.lua b/src/game.lua
index cb34469..5b20492 100644
--- a/src/game.lua
+++ b/src/game.lua
@@ -1,13 +1,9 @@
 
 
-
-
 state = {}
-local function get_initial_game_state(seed)
-    local STARTING_MONEY = 50
-    local map, world = random_map(seed)
+
 -- top right display types
-TRDTS = {
+local TRDTS = {
     NOTHING         = 0,
     CENTERED_EVENQ  = 1,
     EVENQ           = 2,
@@ -18,6 +14,10 @@ TRDTS = {
     TILE            = 7,
 }
 
+local function get_initial_game_state(seed)
+    local STARTING_MONEY = 50
+    local map, world = random_map(seed)
+
     return {
         map = map,              -- map of hex coords map[x][y] to some stuff at that location
         world = world,          -- the root scene graph node for the game 'world'
@@ -68,6 +68,10 @@ local function get_top_right_display_text(display_type)
 end
 
 local function can_do_build(hex, tile, tower_type)
+    if making_hex_unwalkable_breaks_flow_field(hex, tile) then
+        return false
+    end
+
     if not can_afford_tower(state.money, tower_type) then
         return false
     end
@@ -157,7 +161,7 @@ local function game_action(scene)
         state.top_right_display_type = (state.top_right_display_type + 1) % #table.keys(TRDTS)
 
     elseif WIN:key_pressed"f2" then
-        WORLD"flow_field".hidden = not WORLD"flow_field".hidden
+        state.world"flow_field".hidden = not state.world"flow_field".hidden
 
     elseif WIN:key_pressed"tab" then
         if WIN:key_down"lshift" then
@@ -321,6 +325,8 @@ function make_hex_cursor(position, radius, color_f)
 end
 
 function game_scene()
+
+
     local score = am.translate(WIN.left + 10, WIN.top - 20) ^ am.text("", "left"):tag"score"
     local money = am.translate(WIN.left + 10, WIN.top - 40) ^ am.text("", "left"):tag"money"
     local top_right_display = am.translate(WIN.right - 10, WIN.top - 20) ^ am.text("", "right", "top"):tag"top_right_display"
diff --git a/src/grid.lua b/src/grid.lua
index bb4c914..cbd6aec 100644
--- a/src/grid.lua
+++ b/src/grid.lua
@@ -73,6 +73,17 @@ function grid_heuristic(source, target)
     return math.distance(source, target)
 end
 
+function making_hex_unwalkable_breaks_flow_field(hex, tile)
+    local original_elevation = tile.elevation
+    -- making the tile's elevation very large *should* make it unwalkable
+    tile.elevation = 10
+
+    local flow_field = generate_flow_field(state.map, HEX_GRID_CENTER)
+    local result = not hex_map_get(flow_field, 0, 0)
+    tile.elevation = original_elevation
+    return result
+end
+
 function grid_cost(map, from, to)
     local t1, t2 = map.get(from.x, from.y), map.get(to.x, to.y)
 
@@ -86,8 +97,12 @@ function grid_cost(map, from, to)
     return epsilon - cost
 end
 
+function generate_flow_field(map, start)
+    return dijkstra(map, start, nil, grid_cost)
+end
+
 function generate_and_apply_flow_field(map, start, world)
-    local flow = dijkstra(map, start, nil, grid_cost)
+    local flow_field = generate_flow_field(map, start)
 
     local flow_field_hidden = world and world"flow_field" and world"flow_field".hidden or true
     if world and world"flow_field" then
@@ -95,14 +110,18 @@ function generate_and_apply_flow_field(map, start, world)
     end
 
     local overlay_group = am.group():tag"flow_field"
-    for i,_ in pairs(flow) do
-        for j,f in pairs(flow[i]) do
-            if f then
-                map[i][j].priority = f.priority
+    for i,_ in pairs(map) do
+        for j,f in pairs(map[i]) do
+            local flow = hex_map_get(flow_field, i, j)
+
+            if flow then
+                map[i][j].priority = flow.priority
 
                 overlay_group:append(am.translate(hex_to_pixel(vec2(i, j)))
-                                     ^ am.text(string.format("%.1f", f.priority * 10)))
+                                     ^ am.text(string.format("%.1f", flow.priority * 10)))
             else
+                map[i][j].priority = nil
+                log('fire')
                 -- should fire exactly once
             end
         end
@@ -128,6 +147,7 @@ function random_map(seed)
     local neg_mask = am.rect(0, 0, HEX_GRID_PIXEL_WIDTH, HEX_GRID_PIXEL_HEIGHT, COLORS.TRUE_BLACK):tag"negative_mask"
 
     local world = am.group(neg_mask):tag"world"
+    local edge_nodes = {}
     for i,_ in pairs(map) do
         for j,noise in pairs(map[i]) do
             local evenq = hex_to_evenq(vec2(i, j))
@@ -137,6 +157,9 @@ function random_map(seed)
                 -- if we're on an edge -- terraform edges to be passable
                 noise = 0
 
+                -- it's nice to also store the coords for edge nodes in a separate table for later
+                table.insert(edge_nodes, vec2(i, j))
+
             elseif j == HEX_GRID_CENTER.y and i == HEX_GRID_CENTER.x then
                 -- also terraform the center of the grid to be passable
                 -- very infrequently, but still sometimes it is not medium elevation