diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..0007b10
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,59 @@
+name: CI
+
+on:
+  # push only on main; PR branches are covered by pull_request. Scoping push this
+  # way avoids the double run (a push to a branch with an open PR otherwise fires
+  # both events).
+  push:
+    branches: [main]
+  pull_request:
+
+# libtile57 (the static lib linked into chartplotter's CGO binary) and the tile57
+# CLI are built with Zig 0.16. The nested submodules carry the IHO S-101 catalogues
+# that are embedded at build time, so checkout must be recursive.
+#
+# chartplotter cross-compiles its CGO binary against this engine with `zig cc` for
+# linux + windows, amd64/arm64 (see chartplotter's release/xbuild). A default
+# `zig build` builds BOTH libtile57.a and the `tile57` CLI exe, so the whole engine
+# — not just the lib — has to compile for every one of those targets. The
+# cross-compile matrix guards that here, in the engine repo, instead of only
+# surfacing downstream when chartplotter tags a release.
+jobs:
+  build-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          submodules: recursive
+      - name: Install Zig
+        uses: mlugg/setup-zig@v2
+        with:
+          version: 0.16.0
+      - name: Build (native)
+        run: zig build -Doptimize=ReleaseFast
+      - name: Test
+        run: zig build test
+
+  cross-compile:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        target:
+          - x86_64-linux-gnu
+          - aarch64-linux-gnu
+          - x86_64-windows-gnu
+          - aarch64-windows-gnu
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          submodules: recursive
+      - name: Install Zig
+        uses: mlugg/setup-zig@v2
+        with:
+          version: 0.16.0
+      # Builds libtile57.a + the tile57 CLI exe for the target. This is exactly
+      # what chartplotter's `xbuild` invokes per target before linking the Go
+      # binary, so a green matrix means the downstream cross-build will link.
+      - name: Cross-compile ${{ matrix.target }}
+        run: zig build -Dtarget=${{ matrix.target }} -Doptimize=ReleaseFast
diff --git a/tools/ascii.zig b/tools/ascii.zig
index 90c3d8a..f7a62ea 100644
--- a/tools/ascii.zig
+++ b/tools/ascii.zig
@@ -122,6 +122,10 @@ pub fn run(io: std.Io, a: std.mem.Allocator, args: []const [:0]const u8) !void {
 // carriage-returns), alternate screen + hidden cursor, terminal re-measured
 // every frame so a resize just repaints.
 fn runAsciiTui(io: std.Io, a: std.mem.Allocator, c: *chart.Chart, lon0: f64, lat0: f64, zoom0: f64, palette: render.resolve.PaletteId, m: *render.resolve.MarinerSettings, ansi: bool, kitty: bool) !void {
+    // The interactive TUI is POSIX-only: std.posix.termios is `void` on Windows,
+    // so gate the whole raw-mode body out at comptime (same idiom as common.zig's
+    // terminalSize). The non-interactive `ascii` render paths stay cross-platform.
+    if (@import("builtin").os.tag == .windows) return usageErr("--tui is not supported on Windows");
     const stdout = std.Io.File.stdout();
     const stdin_fd = std.Io.File.stdin().handle;
     const old = std.posix.tcgetattr(stdin_fd) catch return usageErr("--tui needs a terminal");
diff --git a/tools/explore.zig b/tools/explore.zig
index f3d7da7..efbd621 100644
--- a/tools/explore.zig
+++ b/tools/explore.zig
@@ -1394,6 +1394,10 @@ fn exLoadCell(
 // transmit-once-per-view + place, deleted each frame — the same cached-region
 // pattern as the ascii kitty TUI, so it never scrolls the layout.
 fn exploreTui(io: std.Io, a: std.mem.Allocator, rows: []const ExIndexRow, cells: []const ExCellSrc, dir: std.Io.Dir, rules: []const u8, F: ExFilters, kitty: bool, palette: render.resolve.PaletteId, m: *const render.resolve.MarinerSettings, source: []const u8) !void {
+    // The interactive TUI is POSIX-only: std.posix.termios is `void` on Windows,
+    // so gate the whole raw-mode body out at comptime (same idiom as common.zig's
+    // terminalSize). The non-interactive `explore` paths stay cross-platform.
+    if (@import("builtin").os.tag == .windows) return usageErr("--tui is not supported on Windows");
     const stdout = std.Io.File.stdout();
     const stdin_fd = std.Io.File.stdin().handle;
     const old = std.posix.tcgetattr(stdin_fd) catch return usageErr("--tui needs a terminal");