Try this fixed code for creating trimesh. Pass it your level mesh.
;creates the mesh to collide with Function ODECollisionMesh(mesh) tris_count% = 0 vert_count% = 0 For i = 1 To CountSurfaces(mesh) tris_count = tris_count + CountTriangles(GetSurface(mesh, i)) vert_count = vert_count + CountVertices(GetSurface(mesh, i)) Next ; DebugLog "Tris count: " + tris_count ; DebugLog "Vertices count: " + vert_count ; DebugLog "surface count: "+CountSurfaces(mesh) vertices_bank% = CreateBank(vert_count * 4 * 4) indices_bank% = CreateBank(tris_count * 3 * 4) offset% = 0 offset_tris% = 0 baseverts=0 For i = 1 To CountSurfaces(mesh) sf = GetSurface(mesh, i) For v = 0 To CountVertices(sf) - 1 PokeFloat vertices_bank, offset, VertexX(sf, v) offset = offset + 4 PokeFloat vertices_bank, offset, VertexY(sf, v) offset = offset + 4 PokeFloat vertices_bank, offset, VertexZ(sf, v) offset = offset + 8 Next For t = 0 To CountTriangles(sf) - 1 For j = 0 To 2 v = TriangleVertex(sf, t, j)+baseverts PokeInt indices_bank, offset_tris, v offset_tris = offset_tris + 4 Next Next baseverts=baseverts+CountVertices(sf) Next t_data = ODE_dGeomTriMeshDataCreate() ODE_dGeomTriMeshDataBuildSimple(t_data, vertices_bank, vert_count, indices_bank, tris_count * 3) Return ODE_dCreateTriMesh(ODE, t_data) End Function